Rollercoaster talks in Vienna towards a nuclear deal entered a 14th day Friday with still no end in sight, as Iran accused the West of back-tracking and Washington said it was prepared to walk away.
A Friday morning deadline to present the deal to the Congress appeared to have been missed, doubling the time for US lawmakers to review the accord -- if it can be reached -- to a potentially risky 60 days.
"Unfortunately we have seen changes in the position and excessive demands... by several countries," Iranian Foreign Minister Mohammad Javad Zarif said late Thursday after praying at a Vienna mosque.
Each of the nations in the group "have different positions which makes the task even harder," Zarif, who again met US Secretary of State John Kerry on Friday morning, told the Iranian television Al-Alam.
The mooted deal with the P5+1 group -- Britain, China, France, Germany, Russia and the United States -- is aimed at preventing Iran from developing a nuclear bomb by scaling down its atomic activities.
In exchange, a painful web of sanctions -- "the most indiscriminate imposed on any nation in human history," Zarif wrote in the Financial Times this week -- would be gradually lifted.
On Thursday, following a meeting with his counterparts from France, Germany and Britain, Kerry said that he would not be rushed into a deal but at the same time that he would not negotiate "forever".
"If the tough decisions don't get made, we are absolutely prepared to call an end to this process," Kerry told reporters.
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.
The current effort to alleviate international concerns about Iran's nuclear programme -- first revealed by dissidents in 2002 -- began in 2013 after moderate Iranian President Hassan Rouhani took power.
In November that year Iran and the powers agreed an interim deal under which Tehran froze parts of its nuclear programme in exchange for minor sanctions relief.
Two deadlines last year -- in July and November -- to turn this into a final accord were missed, but in April in Lausanne, Switzerland the parties managed to agree on the main outlines of a deal.
Armies of experts have made huge progress on some of the thorniest issues needed to turn this framework into a complex final document of possibly up to 100 pages long.
The main text and five complicated technical annexes are all but written, diplomats say.
"The text is done. It's already there. It's a matter of yes or no," EU foreign policy chief Federica Mogherini told CNN on Thursday.
"We are very close, but if the important, historical decisions are not made in the next hours we won't have an agreement."
But there are still gaps, and diplomats say that the remaining differences can only be decided at a political level.
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 from Russia, threw Moscow's weight behind Tehran on Thursday, saying Moscow was "in favour of lifting the embargo as soon as possible."
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".