Image copyright Reuters Image caption Fighting has been raging in the east between the Ukrainian military and pro-Russian rebels 
 
 Ukrainian President Petro Poroshenko has told President Vladimir Putin that reports of Russian tanks moving into Ukrainian territory are unacceptable. 
 
 Ukraine's interior minister said earlier that three tanks had entered rebel areas of the east, and fighting was under way. 
 
 Arsen Avakov said that the tanks entered Ukraine along with other armour through a border checkpoint controlled by rebels in the Luhansk region. 
 
 Russia denied the report. 
 
 The foreign ministry said it was "another fake piece of information". 
 
 Meanwhile, a vehicle belonging to a separatist leader was blown up in the eastern city of Donetsk late on Thursday. 
 
 The BBC's Steve Rosenberg says the van belonged to Denis Pushilin, the self-declared leader of the Donetsk People's Republic. 
 
 Mr Pushilin was not inside but pro-Russian separatists, who control the city, said four people were injured. 
 
 It is not clear what caused the explosion. 
 
 Image copyright AP Image caption A vehicle belonging to a separatist leader was destroyed in an explosion in Donetsk on Thursday 
 
 Image copyright AFP Image caption Denis Pushilin, the self-declared leader of the Donetsk People's Republic, was not in the vehicle at the time 
 
 Earlier, Moscow accused President Poroshenko of not delivering on his pledge to stop fighting in the east. It wants an investigation into allegations that Ukrainian forces used banned incendiary bombs against civilians. 
 
 "The lack of any progress whatsoever in efforts to stop the violence and halt military operations... is causing increasing concern," Russian Foreign Minister Sergei Lavrov said. 
 
 He said that reports in the Russian media that incendiary bombs were being used was another "cause for special concern". 
 
 The bombs are designed to trigger major firestorms and were often used during the Vietnam War before they were banned by the United Nations. 
 
 Ukraine has denied using the weapons. It has accused Russia of allowing the rebels to deploy three Soviet-era T-72 tanks across the border into the east of the country. 
 
 Ukraine's presidency said in a statement that Mr Poroshenko, who was elected last month, had a "substantial" conversation by telephone with Mr Putin on Kiev's plan of bringing peace to the east. 
 
 'Destroyed' 
 
 Mr Avakov said the tanks had crossed the border from Russia along with armoured personnel carriers and artillery pieces in the Dyakove area of Luhansk region, before moving into the neighbouring Donetsk region. 
 
 Two headed towards the town of Horlivka and were attacked by government forces, he added. 
 
 Image copyright Reuters Image caption Rebel fighters could be seen in Snizhne on Thursday 
 
 Unverified video has been posted on YouTube of a battle tank rolling down a street said to be in Snizhne. The footage was shot from a flat overlooking the street. 
 
 A second clip, also said to have been recorded in Snizhne, shows a similar tank passing a roundabout. 
 
 There was no confirmation about the tanks from the insurgents. 
 
 Pro-Russian separatists in the Donetsk and Luhansk regions declared independence after holding referendums last month which were deemed illegal by the government in Kiev. 
 
 Ukraine and the West accuse Russia of supporting and arming the rebels - a claim denied by Moscow. 
 
 The rebellion began amid the turmoil which followed February's ousting of President Viktor Yanukovych, whose last-minute decision not to sign a landmark treaty with the European Union in November sparked mass street protests in Kiev. ||||| KIEV Ukraine accused Russia on Thursday of allowing separatist rebels to bring three tanks and other military vehicles across the border into the east of the country to fight the Ukrainian army. 
 
 Evidence that Russia is sending in weapons or assisting the rebels militarily would implicate Moscow in the uprising against Kiev's pro-Western leaders, making a mockery of its denials that it has played a role in weeks of fighting. 
 
 Interior Minister Arseny Avakov stopped short of directly accusing Russia of sending the tanks but made clear he held President Vladimir Putin responsible for failing to carry out a promise to tighten controls at the border. 
 
 In a sign of his concern, President Petro Poroshenko discussed the situation with his defence and security chiefs and then told Putin by phone that the situation was "unacceptable", his spokesman said. 
 
 Reuters correspondents saw two tanks in the border town of Snizhnye in east Ukraine but said it was not clear where they had come from or whether they had previously been used by the Russian or Ukrainian army. They had no identifying markings to show whether they were Russian army tanks. 
 
 "We have observed columns passing with armoured personnel carriers, other armoured vehicles and artillery pieces, and tanks which, according to our information, came across the border and this morning were in Snizhnye," Avakov told reporters in Kiev. 
 
 He said the columns had come across the border at a checkpoint or checkpoints seized by rebels "despite the Russian Federation's statements that it welcomes the peace process and that the order has been given to strengthen border controls". 
 
 One column, Avakov said, had entered Ukraine in the Dyakove area of the Luhansk region before moving into the neighbouring Donetsk region. Ukrainian forces had confronted the column. 
 
 "There is fighting going on," he said. "Part of the column has been destroyed." 
 
 HIGH STAKES 
 
 Russia did not immediately respond to the accusations and it was not clear how Putin reacted to Poroshenko by phone. His country has already been hit by U.S. and European Union sanctions over events in Ukraine and could face more. 
 
 Confirmation of direct Russian involvement in the rebels' uprising would raise the stakes in Moscow's worst standoff with the West since the Cold war ended more than two decades ago. 
 
 The separatists, who rose up after Poroshenko's predecessor was toppled and fled to Russia in February, deny receiving anything but medical supplies, food and clothing from Russia. "Russia is helping, of course, with humanitarian aid, food, things, medicine, gear. We won't refuse that," a rebel said. Russia has repeatedly denied providing military support to the rebels who have taken control of several towns and cities in east Ukraine and hope that Russia will annexe the region, as it did the Black Sea peninsula of Crimea in March. 
 
 Foreign Minister Sergei Lavrov had earlier on Thursday repeated Moscow's view that the onus for ending violence lay with Ukraine because it has launched a military operation against the rebels. 
 
 But he backed Poroshenko's efforts to push through a peace plan the president has drawn up and discussed with Putin. 
 
 "So far, hope remains that President Poroshenko's statements about the end of violence will be implemented and the talks start," Russia's Interfax news agency quoted Lavrov as saying. 
 
 Few details of the phone call between Putin and Poroshenko were available but the latter's spokesman wrote on Facebook that the Ukrainian leader put a heavy emphasis on the tank movements. 
 
 The presidents met for 15 minutes during World War Two anniversary events in France last week, but it was their first known conversation since Poroshenko was sworn in on Saturday. 
 
 Poroshenko has also been having meetings with a Russian envoy in Kiev and his aides say progress has been made, but talks on a long-running gas-pricing dispute have stalled. 
 
 Alexei Miller, the chief executive of Gazprom, made clear the state-run natural gas exporter would not extend a deadline for Kiev to pay its huge has debts for a third time to allow more time to reach a deal at talks. 
 
 He said a Monday deadline would stand, and Gazprom would cut supplies to Ukraine if it did not to pay off $1.95 billion of its gas debts by then. Cutting supplies to Kiev could disrupt deliveries to the European Union, which gets about a third of its gas imports from Russia, half of them via Ukraine. 
 
 (Additional reporting by Pavel Polityuk in Kiev, Thomas Grove in Snizhnye, and Vladimir Soldatkin in Moscow; Writing by Timothy Heritage; editing by Janet McBride)
– Three Russian tanks have made their way into eastern Ukraine by way of a rebel-controlled checkpoint, according to Ukraine's interior minister, the BBC reports. Arsen Avakov says the tanks, along with armored troop carriers, entered via the Luhansk region then rolled into the adjacent Donetsk region, where they made way to the town of Snizhne; a Reuters correspondent reports seeing three tanks there. Per Avakov, two have since moved to the town of Horlivka, where Ukrainian forces engaged them and a "fight is underway." Russia has not commented on Avakov's accusations that it let the tanks cross.