"We have not discussed an extension. We believe in keeping the pressure on ourselves," the senior US State Department official said after six hours of "very intense" discussions in Vienna.

"If you take the pressure off yourself, then you never have to make hard decisions. Deadlines help people to make hard decisions, and there are hard decisions to be made here. And we must."

She added: "Everyone has been working incredibly hard… these are incredibly complex negotiations, the detail is extraordinary.

"Until everything is agreed, nothing is agreed, and you can get 98 percent of the way, and the last two percent may kill the entire deal."

It was unclear whether Kerry would resume his talks with Iranian Foreign Minister Mohammad Javad Zarif on Thursday, when Zarif was scheduled to meet with negotiators from six world powers in the Austrian capital.

Iran and the six world powers have less than six weeks, until November 24, to strike a comprehensive accord meant to prevent Tehran from developing nuclear weapons under the cover of its civilian atomic programme.

Iran, reeling from sanctions, denies seeking to build the atomic bomb and says it wants to expand its nuclear programme in order to generate electricity and help cancer patients.

But the powers — the United States, France, Britain, Russia, China and Germany — are pressing Tehran to reduce its activities in order to make any dash to make a weapon all but impossible, offering sanctions relief in return.

Last November, the two sides agreed an interim deal and set a July 20 target to agree a lasting accord, but after drawn out talks they gave themselves four more months.

Progress appears to have been made on changing the design of a new reactor at Arak so that it produces less weapons-grade plutonium, as well as on enhanced UN inspections and on the fortified Fordo facility.

The main bone of contention however remains Iran's enrichment capacity, a process rendering uranium suitable for power generation but also, at high purities, for a nuclear weapon.

Other thorny areas include the pace at which sanctions would be lifted, the timeframe that an accord would cover, and a stymied UN probe into past suspect "military dimensions" of Iran's activities.

More time on the clock

Many analysts have begun to believe that the deadline might be extended again, maybe locking in measures related to Arak and Fordo.

"A fully-fledged agreement by November 24 no longer appears likely. What is still possible is a breakthrough that could justify adding more time to the diplomatic clock," Ali Vaez from the International Crisis Group told AFP.

Russian Foreign Minister Sergei Lavrov said on Monday in Paris — after meeting Kerry there — that the November deadline was not "sacred", in the strongest suggestion yet from one of the P5+1 powers.

Zarif earlier Wednesday too appeared to indicate that more time might be needed in order to discuss what he called "serious and innovative" — but unspecified — "new methods".

"It is possible that more time might be needed to discuss these solutions," he told state television late Tuesday after talks with US and EU negotiators including Ashton.

And Iranian President Hassan Rouhani, who has sought to mend fences with the West since coming to power in 2013, has also indicated such a move.

"Our will is that in 40 days the matter will be resolved. But if other things happen and we are not able to solve all the problems, the two camps will find a solution," Rouhani said on state television on Monday.