Global powers seeking to hammer home a deal to curtail Iran's suspect nuclear programme missed another deadline Tuesday, with all sides vowing to now keep working until the end of the week.
"We are continuing to negotiate for the next couple of days," EU foreign policy chief Federica Mogherini told reporters during a break from the negotiations in Vienna, effectively stopping the clock.
But the United States said the terms of a November 2013 interim accord under which Iran has been cutting back its stock of enriched uranium in return for sanctions relief would be extended until Friday, July 10, meaning this is the effective new deadline.
It was the fifth time since 2013 -- and the second time in this round of talks -- that the so-called P5+1 and Iran have missed their own target date to strike an unprecedented accord which has become bogged down over a number of weighty issues.
Mogherini admitted the seven nations at the negotiating table were "interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalise the agreement".
But she said it was still possible to overcome the remaining differences and reach a deal to draw a curtain on a 13-year standoff with Iran, first triggered when dissidents revealed its nuclear programme in 2002.
US Secretary of State John Kerry would remain in Vienna with Mogherini and their Iranian counterpart Mohammad Javad Zarif, the US delegation spokeswoman Marie Harf said.
"We are taking these negotiations day to day to see if we can conclude a comprehensive agreement," she said in a statement.
"We've made substantial progress in every area, but this work is highly technical and high stakes for all of the countries involved. We're frankly more concerned about the quality of the deal than we are about the clock," Harf added.
After talking deep into the night Monday, foreign ministers from the so-called P5+1 -- Britain, China, France, Germany, Russia and the United States -- met twice Tuesday without their Iranian counterparts.
They are seeking to hammer out a final accord to put an atomic bomb out of Iran's reach in return for lifting a web of sanctions, after reaching a framework deal in April.
- Anger at arms embargo -
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"We have no deadline," a spokesman for the Iranian delegation told AFP.
For many observers July 9 had always been the real deadline, and the US team now has its back against the wall trying to nail down the final details by then.
If Kerry fails to hand over a deal by late Thursday, US lawmakers will get 60 days instead 30 to review and vote on it, which risks further complicating its implementation.
Despite progress on a series of complicated annexes, negotiations have hit hurdles on how to ease sanctions against Iran, probing allegations that in the past Tehran sought to develop nuclear arms, and ensuring Iran can continue to have a modest, peaceful nuclear programme.
Russian Foreign Minister Sergei Lavrov confirmed however that at the 11th hour a new problem had emerged -- the lifting of a UN conventional arms embargo on Iran.
"I can assure you that there remains one major problem that's related to sanctions: this is the problem of an arms embargo," Lavrov told Interfax from Vienna.
Under the resolution, nations are banned from selling convention weapons such as tanks and missiles to Tehran. And Russia is believed to be eager to start selling such arms again.
An Iranian official revealed Monday that Tehran was pushing for removal of any mention of a UN arms embargo in any final agreement.
"There is no evidence that the arms embargo has any relation with the nuclear issue," the official said.
A fact sheet put out by the State Department as part of the April framework said that under the final deal there would be a new UN Security Council resolution to extend "important restrictions on conventional arms and ballistic missiles".
Western officials are uneasy about allowing Iran to buy conventional weapons when it is accused of fomenting unrest in the Middle East, even though it is also fighting Islamic State militants, a common enemy for Washington and Tehran.