Iran lashed out at Western powers, accusing them of changing positions at the eleventh hour in intense nuclear talks, as the EU warned on Friday that it was time to say "yes or no" to a deal on the table.

With a 13-year international standoff over Iran's suspect nuclear program 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 backtracking on previous commitments.

"Unfortunately, we have seen changes in the position and excessive demands ... by several countries," Zarif said after praying late at night in a mosque in Vienna.

The emerging deal between Iran and the P5+1 group - China, Britain, France, Germany, Russia and the United States - is aimed at preventing Teheran 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.

The top US diplomat said that if the "tough decisions" were not made soon, he was prepared to walk away.

Test for the decades

Talks to alleviate international concerns about Iran's nuclear program - 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 said.

"We are very close, but if the important, historical decisions are not made in the next hours, we won't have an agreement," she said.

Negotiators are said to have made huge progress on some of the thorniest issues, including a mechanism to unlock the 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 the end of Thursday, Washington time, was also 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.