The U.S. stepped up efforts to dissuade Russia from intervening in Ukraine's increasingly unstable east, with President Barack Obama telling President Vladimir Putin Monday that a diplomatic solution to the crisis is still possible even as he warned against further escalation. 
 
 Russia requested the call, the first between the presidents since March 28, after Mr. Putin annexed Crimea. 
 
 The White House also confirmed that... ||||| SLAVIANSK/DONETSK, Ukraine Armed pro-Russian separatists seized more buildings in eastern Ukraine on Monday, expanding their control after the government failed to follow through on threatened military crackdown leaving Moscow's partisans essentially unopposed. 
 
 European foreign ministers agreed to widen sanctions against Moscow and the White House said Washington was seeking ways to impose more "costs" on Russia, for what Kiev and its Western friends call a Russian plot to dismember Ukraine. 
 
 Rebels in the town of Slaviansk, where the authorities failed to follow through with their announced "anti-terrorist" operation, called for Russian President Vladimir Putin's help. 
 
 Ukraine's interim president Oleksander Turchinov said on Monday the offensive against the rebels would still go ahead. But in a sign of discord behind the scenes in Kiev, he sacked the state security chief in charge of the operation. 
 
 In one of the first signs of a military deployment by Kiev's forces, a Ukrainian column of two tanks and more than 20 armored personnel carriers packed with paratroops was seen about 70 km (50 miles) northwest of Slaviansk on Monday evening, according to video journalist Maksim Dondyuk who filmed them. 
 
 In Donetsk, rebels holed up in the administrative headquarters of a province that is home to 10 percent of Ukraine's population said they planned to seize control of infrastructure and the levers of state power. They have declared an independent "People's Republic of Donetsk" and sought Putin's protection if they are attacked. 
 
 Rebels have also seized buildings in around 10 other towns and cities across other eastern provinces which form the heartland of Ukraine's heavy industry. 
 
 In a bid to undercut the rebels' demands, Turchinov held out the prospect of a countrywide referendum on the future shape of the Ukrainian state. Pro-Russian secessionists want separate referendums in their regions, which Kiev says is illegal. 
 
 The uprising in eastern Ukraine began eight days ago but has accelerated sharply in the past 48 hours, with separatists seizing ever more buildings, including arsenals filled with weapons. They have met little opposition. 
 
 Kiev says the separatists are organized by Moscow, seeking to repeat the seizure of the Crimea region, which Moscow occupied and annexed last month. 
 
 Russia says the armed men are all locals acting on their own, but Western officials say the uprising is too well-coordinated to be entirely spontaneous, and bears too many similarities to the Russian operation in Crimea. 
 
 "I don't think denials of Russian involvement have a shred of credibility," British Foreign Minister William Hague said, before a meeting with EU counterparts. 
 
 Hague later announced that the ministers had agreed to expand a list of Russians barred from travelling or doing business in the EU. Work would begin to come up with new names for the sanctions list, Hague said. 
 
 In Washington, White House spokesman Jay Carney said President Barak Obama would speak to Putin by phone later on Monday. Washington is also planning to expand its sanctions list. Russia has so far shrugged off targeted sanctions. 
 
 Moscow says it has the right to intervene to protect Russian speakers in Ukraine, and has portrayed the people of the east as under threat from gangs of Ukrainian-speaking "fascists". NATO says Russia has tens of thousands of troops massed on the frontier, able to capture eastern Ukraine within days. 
 
 ULTIMATUM EXPIRES 
 
 Turchinov had threatened to launch a military crackdown by 9 a.m., but as the deadline expired there was no sign of any action in Slaviansk. A rebel leader, in an appeal issued through journalists, asked Putin to "help us as much as you can". 
 
 The Kremlin said the Russian president was listening. 
 
 "Unfortunately, there's a great many such appeals coming from the Eastern Ukrainian regions addressed directly to Putin to intervene in this or that form," spokesman Dmitry Peskov said. "The president is watching the developments in Eastern Ukraine with great concern." 
 
 Also in Slaviansk, about 150 km (90 miles) from the Russian border, a small airfield which was occupied by Ukrainian air force planes on Sunday was empty on Monday and pro-separatist forces said they were now in control of it. 
 
 Eastern Ukraine seems to be rapidly spinning out of the control of the central government. The governor of Donetsk, a multi-millionaire appointed by Kiev, has not been seen since April 11. A man calling himself Donetsk's new police chief has appeared wearing the orange and black separatist ribbon. 
 
 The Ukrainian defense ministry acknowledged that it has had difficulty mobilizing the armed forces in the east, where some units have been blockaded in by rebellious locals. 
 
 "On some occasions we have lost the information war and there have been blockades of our units. People don't understand why they are coming," said acting Defence Minister Mykhailo Koval. He said 26 members of a reconnaissance unit had been blockaded for the past day and a half in Slaviansk. 
 
 "Negotiations are under way to free them to allow them to link up with our main force." 
 
 In the town of Horlivka about 100 pro-Russian separatists attacked the police headquarters on Monday. Video footage on Ukrainian television showed an ambulance treating people apparently injured in the attack. 
 
 Russia's foreign ministry called Turchinov's planned military operation a "criminal order" and said the West should bring its allies in Ukraine's government under control. 
 
 Turchinov's website said he told U.N. Secretary-General Ban Ki-moon he would welcome U.N. peacekeepers in Ukraine. The proposal was rhetorical as no such deployment has been proposed or could ever take place over Russia's Security Council veto. 
 
 The Ukraine crisis has led to the biggest confrontation between Moscow and the West since the Cold War. Washington said a Russian fighter aircraft had made 12 low altitude passes over a U.S. warship in the Black Sea over the weekend, which it called a "provocative and unprofessional Russian action". 
 
 Outside the Slaviansk city council offices stood a group of about 12 armed men in matching camouflage fatigues with black masks, one of whom was holding a Russian flag. 
 
 They said they were Cossacks - paramilitary fighters descended from Tsarist-era patrolmen - but did not say where from. One told Reuters: "The borders between Ukraine, Russia and Belarus are artificial and we are here to take them away." 
 
 In Donetsk, leaders of the self-declared "People's Republic" held a strategy meeting to plan their seizure of control of the rest of the region's state functions. 
 
 "Everything from city cleaning to the sewage system, the airport, railway stations, military... should be under your control," one leader, Vladimir Makovich, told about two dozen other senior separatists in a dark room on the top floor of the 11-storey government headquarters. 
 
 Over the past week, the rebels have turned the massive Soviet-era building into a bastion for urban warfare. Barricades crisscross the corridors and steel plates are welded to windows. 
 
 "We are ready for storming at any time. No matter what happens, this building will not be given up," said Alexander Zakharchenko, 38, commander of a paramilitary unit made up of members of a martial arts club. 
 
 Turchinov's announcement he was sending in the army was the first time the military has been activated in six months of internal disorder. The plan implies a lack of confidence in the 30,000-strong interior ministry troops, partly discredited by identification with ousted president Viktor Yanukovich. 
 
 Russian stocks and the ruble fell sharply on Monday, reflecting fears of further Russian military intervention in Ukraine and more western sanctions against Moscow. 
 
 Kiev is also facing economic disarray. The central bank nearly doubled its overnight interest rate to 14.50 percent from 7.50 percent. Ukraine's hryvnia currency has lost 38 percent of its value against the dollar this year. 
 
 Moscow has largely brushed off sanctions so far, which the United States and Europe have explicitly designed to target only a limited number of officials and avert wider economic harm. 
 
 (Additional reporting by Natalia Zinets and Richard Balmforth in Kiev; Writing by Peter Graff) ||||| Media playback is unsupported on your device Media caption The BBC's Olga Ishvina reports from inside a police department in Gorlovka, near Donetsk, where police and pro-Russian activists are "cohabiting" 
 
 Pro-Russian activists have attacked another official building in east Ukraine, ignoring a deadline to leave or face eviction by Ukrainian forces. 
 
 A crowd stormed a police station in the town of Horlivka, near Donetsk, taking control of the building. 
 
 Ukraine's interim president hit out at "aggression" from Russia, but signalled support for a national referendum. 
 
 Olexander Turchynov said Kiev was "not against" a vote on the future of the country, a key demand from protesters. 
 
 Mr Turchynov also said Ukraine was preparing an "anti-terrorist operation" against gunmen occupying government buildings in Sloviansk and a number of other towns and cities. 
 
 His office said that he had suggested the UN could assist in any such operation. This would be highly unlikely as Russia has a veto on the Security Council, which would have to authorise any such action. 
 
 Correspondents say people in eastern Ukraine are anxiously waiting to see if Mr Turchynov carries through on his threat to use the army against the pro-Russian groups. 
 
 Image copyright AFP Image caption A pro-Russia crowd threw rocks at the police HQ in Horlivka before storming it 
 
 Image copyright AFP Image caption Pro-Russian gunmen continued to patrol the streets of Sloviansk on Monday 
 
 Image copyright AFP Image caption There were rival pro-Russian and pro-Ukrainian rallies in the town of Kharkiv on Sunday 
 
 In other developments: 
 
 Moscow denied allegations that Russian agents had been fomenting unrest in east Ukraine 
 
 Vitaly Tsyhanok, the head of Ukraine's anti-terror operations, was sacked after criticism of Kiev's response to the crisis in the east 
 
 A Ukrainian official said the government had not declared a state of emergency over the crisis in the east because it would require suspending the run-up to presidential elections on 25 May 
 
 Britain called on the European Union to agree "further sanctions" against Russia in response to the escalating crisis 
 
 'A united Ukraine' 
 
 Media playback is unsupported on your device Media caption This video, posted on YouTube, shows a man who claims to be a Russian soldier instructing police officers in Horlivka, Ukraine 
 
 In a televised address to parliament, interim President Turchynov suggested Kiev would be open to moving from a republic into a federation and giving broader rights to Ukraine's Russian speakers. 
 
 Crisis timeline Nov 2013: President Viktor Yanukovych abandons an EU deal 
 
 President Viktor Yanukovych abandons an EU deal Dec: Pro-EU protests erupt 
 
 Pro-EU protests erupt 20-21 Feb 2014: Dozens killed in Kiev clashes 
 
 Dozens killed in Kiev clashes 22 Feb: Mr Yanukovych flees; 
 
 Mr Yanukovych flees; 27-28 Feb: Pro-Russian gunmen seize key buildings in Crimea 
 
 Pro-Russian gunmen seize key buildings in Crimea 16 Mar: Crimea voters choose to secede in disputed referendum: Russia later absorbs region 
 
 Crimea voters choose to secede in disputed referendum: Russia later absorbs region Apr: Pro-Russia activists take over government buildings and police stations in eastern Ukraine What is happening where? Crisis timeline 
 
 The pro-Russian groups who have seized government buildings in eastern regions are demanding local referendums on either increased local rights or an option to join the Russian Federation. 
 
 But Mr Turchynov stopped well short of giving in to these demands by showing support for a national referendum, of which the outcome is uncertain because most people in Kiev and the Ukrainian-speaking west reject the idea of federalisation. 
 
 "We are not against holding a national referendum," Mr Turchynov said. "I am certain that a majority of Ukrainians will support an indivisible, independent, democratic and united Ukraine." 
 
 He also used the speech to accuse Moscow of open aggression in the east of the country. 
 
 "It's not a confrontation between Ukrainians, but covert and now no longer covert aggression by the Russian Federation against our country," he added. 
 
 Media playback is unsupported on your device Media caption Eastern Ukraine is "tense with anticipation", reports David Stern in Donetsk 
 
 Russian Foreign Ministry Sergei Lavrov said it was not in Russia's interests for Ukraine to break up but added that Moscow wanted all citizens of the country to be given equal treatment by Kiev. 
 
 He also denied allegations that Russian agents had been fomenting unrest in eastern Ukraine and said he was seeking explanation from US of reports that CIA director John Brennan had visited Kiev. 
 
 Media playback is unsupported on your device Media caption Ukrainian Deputy Foreign Minister Danylo Lubkivsky said special forces were trying to clear pro-Russian protesters without causing casualties 
 
 Russian President Vladimir Putin's spokesman told Interfax he had been "receiving many appeals from eastern Ukraine... calling on him to help in some form", and that he was watching developments "with a great deal of concern". 
 
 At an emergency meeting of the UN Security Council on Sunday, Russia urged Kiev not to use force against protesters in eastern Ukraine. 
 
 The Russian Ambassador to the UN, Vitaliy Churkin, called on the government in Kiev to "start a genuine dialogue". 
 
 Mr Churkin warned UN diplomats that there were neo-Nazis and anti-Semites within the ranks of "the self-proclaimed government in Kiev". 
 
 He said the opinions and interests of protesters in eastern Ukraine had not been taken into consideration. 
 
 "The henchmen of Maidan [protest movement] must stop attacking their own people," said Mr Churkin. 
 
 Media playback is unsupported on your device Media caption Russia calls on Ukraine's government to cease a "war with its own people" 
 
 Media playback is unsupported on your device Media caption Ukrainian ambassador to the UN asks for Russia to "leave us in peace" 
 
 But Ukraine's UN ambassador Yuriy Sergeyev said Moscow had artificially created the crisis in the east of the country. 
 
 Ukraine's foreign ministry said on Monday that it had proof Russian forces had been behind the "separatist operation" and said it would present its evidence at a meeting in Geneva later this week. 
 
 The assistant UN Secretary General had warned that Ukraine "teeters on the brink". 
 
 European Union foreign ministers meeting in Luxembourg to discuss the crisis approved measures to provide up to 1bn euros (£800mn; $1.4bn) of economic assistance to Ukraine. 
 
 The ministers also approved "the temporary reduction or elimination of customs duties" for Ukrainian exports to the EU. 
 
 Also on Monday, the US signed a $1bn loan guarantee agreement for Ukraine after a meeting between US Treasury Secretary Jacob Lew and his Ukrainian counterpart, Oleksandr Shlapak, in Washington.
– Ukraine's interim president gave pro-Russia militants occupying government buildings in Slaviansk and other eastern Ukraine areas until this morning to get out, or be forced out by the Ukrainian military—and the deadline has passed without the militants leaving. So far, Oleksandr Turchinov has not sent in the army, but clashes between Ukrainian forces and pro-Russia gunmen on the outskirts of the city left at least one Ukrainian dead yesterday, the BBC reports. Today, at least 100 pro-Russia separatists attacked a police station in another eastern Ukraine city, Reuters reports. At yesterday's emergency UN Security Council meeting, Russia's ambassador urged the Ukrainian government to "start a genuine dialogue" with protesters in eastern Ukraine rather than using force, while Ukraine's ambassador accused Russia of being behind the "separatist operation" in the first place. If Kiev follows through on its threat to bring in the military, Russia, which has 40,000 troops on Ukraine's eastern border, could push back; the US has compared the events in eastern Ukraine to the ones that led up to Russia's annexation of Crimea. But the Wall Street Journal reports that Turchinov also said today he's open to holding a referendum on the possibility of giving certain regions more autonomy, which is one of the rebels' demands.