Iran lashed out at Western powers accusing them of changing positions at the 11th hour in intense nuclear talks, as the EU warned it was time Friday to say "yes or no" to a deal on the table.
With a 13-year international standoff over Iran's suspect nuclear programme coming to a head, global powers leading the negotiations sought to ramp up the pressure for a deal.
But Iranian Foreign Minister Mohammad Javad Zarif hit back, saying Western countries among the so-called P5+1 group on the other side of the talks were back-tracking on previous commitments.
"Unfortunately we have seen changes in the position and excessive demands... by several countries," said Zarif after praying late at night in a mosque in Vienna.
The emerging deal between Iran and the P5+1 group -- Britain, China, France, Germany, Russia and the United States -- is aimed at preventing Tehran from developing a nuclear bomb in exchange for relief from a web of biting international sanctions.
Each of the nations in the group "have different positions which makes the task even harder," Zarif told the Iranian television Al-Alam.
As this round of talks lurched into a 14th day in the Austrian capital, US Secretary of State John Kerry insisted he would not be rushed into a deal -- but warned he would not stay at the negotiating table forever.
If the "tough decisions" were not made soon, the top US diplomat said he was prepared to walk away.
Talks to alleviate international concerns about Iran's nuclear programme -- first revealed by dissidents in 2002 -- resumed in earnest in September 2013 after the election of moderate Iranian President Hassan Rouhani.
After two tortuous years of negotiations between Kerry and Zarif, there is hope that a climax may be merely hours away.
"The text is done. It's already there. It's a matter of yes or no," EU foreign policy chief Federica Mogherini told CNN.
"We are very close, but if the important, historical decisions are not made in the next hours we won't have an agreement."
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- Test for the decades -
Negotiators are said to have made huge progress on some of the thorniest issues, including a mechanism to unlock a web of biting sanctions and ways to probe allegations that Iran has sought to develop nuclear weapons in the past.
The main text and five complicated technical annexes are all but written.
Two deadlines for a deal have already been passed in this round of talks, and a third target date set by US lawmakers to receive a copy of a deal by midnight Washington time (0400 GMT) was also set to be missed.
"The stakes are very, very high, we will not rush and we will not be rushed," Kerry vowed, even though the US Congress is now set to get 60 days instead of 30 to review any deal.
Kerry stressed negotiators were focusing on the quality of the deal, which "has to be one that can withstand the test of time".
"It is not a test of a matter of days or weeks or months. It's a test for decades," he said.
Iran's demand that a UN arms embargo be lifted has thrown a spanner in the works in recent days. Western nations have baulked at the idea, as Tehran is accused of fomenting violence in the Middle East.
But Russian Foreign Minister Sergei Lavrov, speaking at a summit of emerging economies in the Russian city of Ufa, threw Moscow's weight behind Tehran.
"We are in favour of lifting the embargo as soon as possible," Lavrov told reporters.
And an Iranian official, who asked not to be identified, said Beijing had also backed Tehran's demand, adding the easing of the UN embargo is a "requirement".
He insisted, however, that "a comprehensive agreement is within reach. It just requires people to abandon unnecessary or illusional objectives".