US Secretary of State John Kerry said Thursday major powers won't be "rushed" into a nuclear deal with Iran, adding he was prepared to walk away from the talks if "tough decisions" are not made soon.

Speaking just hours from a deadline to present a deal to the US Congress, Kerry told reporters in Vienna that because "the stakes are very, very high, we will not rush and we will not be rushed."

"We're here because we believe we're making real progress", Kerry said.

But he warned "we are not going to sit at the negotiating table forever."

"If the tough decisions don't get made, we are absolutely prepared to call an end to this process."

French Foreign Minister Laurent Fabius, announcing he would stay in the Austrian capital Thursday night to continue the talks, said meanwhile that "things are going in the right direction".

British Foreign Secretary Philip Hammond also stayed in Vienna, while his German counterpart Frank-Walter Steinmeier was returning to Berlin. Sources said he would be back early on Friday.

"We are very close, but if the important, historical decisions are not made in the next hours we won't have an agreement", EU foreign policy chief Federica Mogherini told CNN.

"On the other side it's clear that for everybody that making the decisions in one week, two weeks, or three weeks from now will not be easier, but more difficult for everybody," she said.

The mooted deal between Iran and the P5+1 group -- Britain, China, France, Germany, Russia and the United States -- is aimed at ending a 13-year standoff by curbing Iran's nuclear activities in exchange for sanctions relief.

If the US Congress does not receive the text by early Friday morning Vienna time -- midnight in Washington -- it will likely delay its implementation by doubling the review period to 60 days.

But an Iranian official insisted to AFP: "For us, no date is sacred if it means sacrificing a good accord."

And Iranian Foreign Minister Mohammad Javad Zarif shouted to reporters from the balcony of the Coburg hotel where the talks were being held: "We will stay as long as necessary."

Two deadlines have already been missed during this round of negotiations alone which have now stretched into their 13th day. Kerry met with Zarif for just under an hour late Thursday, US officials said.

- Spanner in the works -

All sides say huge progress has been made in the past days of negotiations -- the final stage of marathon talks first launched in September 2013 -- with most of the accord written.

"The text is done. It's already there. It's a matter of yes or no... the parties have the political space to take political decisions," Mogherini said.

Some of the thorniest issues such as a time frame for lifting sanctions and a UN probe into allegations that Iran in the past sought to develop nuclear weapons, also appear close to resolution.

But Iran's demand that a UN arms embargo be lifted has thrown a spanner in the works. Western nations have balked at the idea, with Tehran still accused of fomenting violence in the Middle East.

Russian Foreign Minister Sergei Lavrov, who was Thursday in the Russian city of Ufa at a summit with emerging economies, threw Moscow's weight behind Tehran on the issue.

"We are in favour of lifting the embargo as soon as possible and will support a decision made by Iran's negotiators," Lavrov told reporters.

The UN Security Council arms embargo had been imposed to force Iran to negotiate, a goal that had "long been reached," he stressed.

Resuming arms deliveries would help Iran combat terrorism and radicals from the Islamic State group, he added.

There were no "insurmountable problems" left to tackle at the talks, unless somebody tried to deliberately stall the negotiations, Lavrov insisted.

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