LAUSANNE, Switzerland — The United States and its negotiating partners on Tuesday extended their deadline for reaching a preliminary accord with Iran on its nuclear program until Wednesday. 
 
 But that one-day extension came with a White House warning that the Obama administration was prepared to step back from its diplomatic efforts if it concluded that the initial accord could not be reached. 
 
 “I actually would say that if we’re not able to reach a political agreement in the timeline that we’ve described that we would walk away from the negotiating table,” said Josh Earnest, the White House spokesman. ||||| High-level negotiations on Iran’s nuclear programme enter their last day on Tuesday before a deadline for agreeing the outline for a comprehensive settlement, with top diplomats still struggling to overcome persistent obstacles. 
 
 The US secretary of state, John Kerry, his Iranian opposite number, Mohammad Javad Zarif, alongside foreign ministers from France, the UK, Germany and China, worked late into the evening on two consecutive nights in an effort to break the deadlock. 
 
 Russia’s Sergei Lavrov left the talks on Monday afternoon to hold official meetings in Moscow, but said he would return to Lausanne later on Tuesday, telling reporters in the Russian capital that there was a good chance of success. 
 
 “The chances are high. They are probably not 100% but you can never be 100% certain of anything. The odds are quite ‘doable’ if none of the parties raise the stakes at the last minute,” Lavrov said on Tuesday. 
 
 A US State Department spokeswoman said on Monday night that prospects of a deal on Tuesday were “50-50”. Working teams of experts and diplomats were instructed to work through the night on outstanding issues in the search for a breakthrough. 
 
 
 
 Diplomats said that the principal sticking point was the issue of UN security council sanctions on Iran. Zarif’s team is sticking by the demand by their country’s supreme leader, Ali Khamenei, for all sanctions to be lifted at once in return for Iranian acceptance of restrictions on its nuclear programme over a period of at least 10 years. 
 
 
 
 The six-nation group is offering several relief measures, lifting the EU oil embargo and removing banking restrictions in moves synchronised with the suspension of corresponding US sanctions. But it insists that some UN sanctions must stay in place until Iran has convinced the international community it has no intention of pursuing a weapons programme, a task that could take many years. 
 
 European diplomats said the final day of talks would show whether Iran’s seemingly immovable stance on UN sanctions was a hardball negotiating tactic, or whether Khamenei had left them no negotiating space. Khamenei said on his website on Monday morning: “Sanctions must be lifted in one go, not as a result of future Iranian actions.” 
 
 Facebook Twitter Pinterest Iran’s nuclear negotiations: how did we get here? – video explainer 
 
 In a further sign of problems at the talks, Khamenei’s foreign policy adviser, Ali Akbar Velayati, cast doubt on the west’s motives at the talks. 
 
 
 
 “Our negotiating team are trustworthy and compassionate officials that are working hard, but they should be careful with the enemies’ deceptive and skilful tactics,” Velayati told Fars news agency. 
 
 A diplomat in Lausanne quoted by the Chinese news agency Xinhua said the atmosphere on Monday had turned from optimism to gloom among negotiators. 
 
 Another longstanding hurdle to an agreement, the question of Iranian research and development, has been partially resolved. Iran has accepted strict limits on its development of new super-efficient centrifuges during the first 10 years of a deal, but there are disagreements on what would happen afterwards. 
 
 
 
 The six-nation group wants restrictions to extend for a further five years, wary that new centrifuges could drastically reduce Iran’s breakout time (the period it would need to build a bomb, should it make that decision). Iran rejects such prolonged curbs on its centrifuge development, saying it would render the country dependent on foreign technology. 
 
 A third problem seems to have been largely resolved in the past few days. Iran wanted to keep some centrifuges spinning at its underground enrichment plant in Fordow. The six powers negotiating with Tehran proposed a compromise in which a few hundred centrifuges spin but do not process uranium. They would purify other elements used for medical and scientific purposes. 
 
 Reza Marashi, research director of the National Iranian American Council, said even if the 31 March deadline was missed, none of the parties at the table would walk away. 
 
 “Both sides stand to lose massively. There is a propensity to focus on minutiae, but what has got lost is how much each side has to lose,” Marashi said. “Are we going to let this collapse over differences on how long Iran isn’t allowed to work on new centrifuges?” ||||| LAUSANNE, Switzerland (AP) — Wrapping up six days of marathon nuclear talks with mixed results, Iran and six world powers prepared Tuesday to issue a general statement agreeing to continue negotiations in a new phase aimed at reaching a comprehensive accord by the end of June, officials told The Associated Press on Tuesday. 
 
 Iranian Foreign Minister Javad Zarif, center right, European Union High Representative Federica Mogherini, center left, and other officials from Britain, China, France, Germany, Russia and the United... (Associated Press) 
 
 Iranian Foreign Minister Javad Zarif, right, European Union High Representative Federica Mogherini, center, and European Union Political Director Helga Schmid, left, wait for the start of a meeting on... (Associated Press) 
 
 U.S. Secretary of State John Kerry, second left, U.S. Secretary of Energy Ernest Moniz, left, British Foreign Secretary Philip Hammond, center, Russian Deputy Foreign Minister Sergei Ryabkov, second right,... (Associated Press) 
 
 U.S. Secretary of State John Kerry, third left, chats with U.S. Under Secretary for Political Affairs Wendy Sherman, as U.S. Secretary of Energy Ernest Moniz, second right, takes a note while waiting... (Associated Press) 
 
 Iranian Foreign Minister Javad Zarif, lefts, greets Ali Akbar Salehi, head of Iran's Atomic Energy Organization, upon the latter's arrival for a meeting on Iran's nuclear program with other officials... (Associated Press) 
 
 U.S. Secretary of State John Kerry waits for the start of a meeting on Iran's nuclear program with officials from Britain, France, Germany, China, the European Union and Iran at the Beau Rivage Palace... (Associated Press) 
 
 The joint statement is to be accompanied by additional documents that outline more detailed understandings, allowing the sides to claim enough progress has been made thus far to merit a new round, the officials said. 
 
 The talks have already been extended twice as part of more than a decade of diplomatic attempts to curb Tehran's nuclear advance, and the next stage will be presented as a new phase, because most of the parties had ruled out another prolongation of this round. 
 
 One of the officials said the statement was general in part because differences between the sides remained ahead of a new phase of negotiations toward a comprehensive deal by late June. The second official said other documents will be more technical in nature and will also be made public later in the day. 
 
 Both demanded anonymity because they are not authorized to comment on the talks. 
 
 American officials earlier had said the sides were aiming for a framework agreement by the end of March but then revised the language, speaking of an "understanding." 
 
 That appeared due in part to opposition to a two-stage agreement from Iran's supreme leader, Ayatollah Ali Khamenei. Earlier this year, he demanded only one deal that nails down specifics and does not permit the other side to "make things difficult" by giving it wiggle room on interpretations. 
 
 The documents were being finalized among the six countries negotiating with Iran, and the Iranian side had not yet signed off on them, said the first official. 
 
 Russian Foreign Minister Sergey Lavrov, who left Lausanne Monday, was heading back to the Swiss city, also indicating that an end to the talks was near. He departed on Monday but said he would return if a deal was imminent. 
 
 In Moscow, he told reporters: "Prospects for this round of negotiations were not bad, and I would even say good." 
 
 Foreign ministers of five nations at the table already joined U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif at the talks over the weekend in an intense effort to reach a political understanding on terms that would curb Iran's nuclear activities in exchange for sanctions relief. 
 
 Kerry and others at the table said the sides have made some progress, with Iran considering demands for further cuts to its uranium enrichment program but pushing back on how long it must limit technology it could use to make atomic arms. In addition to sticking points on research and development, differences remain on the timing and scope of sanctions removal, the officials said. 
 
 The Obama administration says any deal will stretch the time Iran needs to make a nuclear weapon from the present two to three months to at least a year. But critics object that it would keep Tehran's nuclear technology intact. 
 
 Officials in Lausanne said the sides were advancing on limits to aspects of Iran's program to enrich uranium, which can be used to make the core of a nuclear warhead. 
 
 Uranium enrichment has been the chief concern in over more than a decade of international attempts to cap Iran's nuclear programs. But Western officials say the main obstacles to a deal are no longer enrichment-related but instead the type and length of restrictions on Tehran's research and development of advanced centrifuges and the pace of sanctions-lifting. 
 
 Over the past weeks, Iran has moved from demanding that it be allowed to keep nearly 10,000 centrifuges enriching uranium, to agreeing to 6,000. The officials said Tehran now may be ready to accept even fewer. 
 
 Tehran says it wants to enrich only for energy, science, industry and medicine. But many countries fear Iran could use the technology to make weapons-grade uranium. 
 
 ____ 
 
 Associated Press television producer Mahdi Fattahi contributed from Tehran
– With a midnight deadline looming, Iran and the six nations it's conferring with in Switzerland have reached a deal—to continue talks for a deal on Tehran's hotly contested nuke program. The nations are expected to release a joint statement today announcing they'll enter a "new phase" of negotiations that will end with a full-fledged agreement by the end of June, according to two unnamed officials, per the AP. While headway had been made, key sticking points mean the language is being revised to say today's talks will end in an "understanding," with "technical" issues to be resolved in coming months, the AP notes. What's going on: Iran wants all UN sanctions lifted immediately; the six-country bloc is willing to make other concessions—including nixing banking limitations and getting rid of the EU oil embargo—but wants some sanctions in place until Iran makes good on its promises. That looks unlikely, with supreme leader Ali Khamenei posting yesterday, "Sanctions must be lifted in one go, not as a result of future Iranian actions," per the Guardian. Although Iran has said it will acquiesce to restrictions on its centrifuge development over the next decade, beyond that is fair game. The other countries at the negotiating table want to cut down Iran's "breakout time" (the time it would take to construct a bomb) by adding five more years onto the 10-year moratorium. Iran argues that would keep the country under the thumb of other nations' technology, the Guardian notes. The question of Iran's uranium stash remains on the table, with Tehran's main negotiator saying Iran has no intention of shipping its fuel out, and the US State Department conceding that there's still a lot of back and forth on this issue. "The bottom line is that we don't have agreement with the Iranians on the stockpile issue," a State rep said yesterday, per the New York Times.