Iran and the US still aim to strike a mammoth nuclear deal by a November 24 deadline, a US official said Wednesday after talks involving US Secretary of State John Kerry that yielded no apparent major breakthrough.

"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.

US not discussing extending Iran talks deadline
Vienna (AFP) Oct 15, 2014 - The US is not discussing extending the deadline to strike a nuclear deal with Iran beyond November 24, a senior US official said Wednesday following comments from other figures suggesting such a move.

"We're not talking about an extension," the official said after US Secretary of State John Kerry arrived in Vienna for talks with his Iranian counterpart aimed at advancing the stalled negotiations.

"We're talking about getting this done by November 24. Everyone is at the table including the Iranians, about wanting to get this done," the senior US State Department official said on condition of anonymity.

The Iranians "know they're not going to get the sanctions relief they need if" there is no deal, the official added.

"There's still time to get this done, there is enough time to get the technical work done, to get the political agreement, to get the annexes (to any deal) done if everyone can make the decisions they need."

Iran and the five permanent members of the UN Security Council plus Germany have been negotiating for months a deal aimed at easing fears that Iran might develop nuclear weapons under the guise of its civilian programme.

Last November they struck an interim deal, buying time to agree a lasting accord. Initially they set themselves a target date of July 20 but this was extended in July by four months until November 24.

Despite progress in some areas, the talks are thought to be snagging in particular on the key question of Iran's future capacity to enrich uranium, leading to speculation among experts of another extension.

The foreign minister of Russia, Sergei Lavrov, one of the six powers, said in Paris on Monday that the deadline was "not sacred" and Iranian officials -- including President Hassan Rouhani -- have made similar comments.

In return for scaling back its nuclear activities, the six powers are offering Iran relief from painful sanctions, and the timing and pace of such relief remains under discussion.

The US official said that Washington had identified sanctions that could be eased in a first step in any deal but indicated that there remained disagreement on this with the Iranians.

The official said the aim of the next two days' talks was "to really see where we are on each of the issues, and what still needs to be done ... (and) talk together about where we are and sort of chart a path for the next five and a half weeks".

"We've been very creative in terms of what we've put on the table, the technical ideas," the official said, stressing however that "creative does not equal concessions".

"We have made some progress on issues... people keep asking about when there's going to be a big breakthrough, but that is not how this has tended to work. It's almost like you're just chipping away, the hard work every day."