Iran said Thursday it is against extending a November 24 deadline for troubled nuclear talks, even though major stumbling blocks remain in the way of a deal.

"We only have 40 days left to the deadline and... none of the negotiators find (an) extension of talks appropriate," Mohammad Javad Zarif said in Vienna, a day after six hours of intense talks with US Secretary of State John Kerry.

"We share this view... and we think there is no need to even think about it," Zarif said in the Austrian capital, quoted by the state television's website.

The comments echoed a senior US State Department official late Wednesday after the talks between Kerry, Zarif and EU foreign policy chief Catherine Ashton in a Vienna hotel room.

"Deadlines help people to make hard decisions, and there are hard decisions to be made here. And we must," the official said on condition of anonymity.

- P5+1 powers -

Kerry left Vienna on Thursday morning -- after problems with his aircraft -- but Zarif remained for talks with political directors from the US, Russia, Britain, China, France and Germany chaired by Ashton.

The deal being sought, after more than a decade of rising tensions, is meant to ease concerns that Iran might be able to develop nuclear weapons under the guise of its civilian programme.

To do this, the "P5+1" powers want Iran to scale down dramatically the scope of its atomic activities, offering in return relief from painful sanctions, but Iran is resisting this.

Iran denies seeking to build the atomic bomb and says it wants to expand its nuclear programme in order to generate electricity and treat cancer patients.

"Diplomatic efforts to find a resolution to the Iranian nuclear issue are now in a critical phase," Ashton's spokesman Michael Mann said late on Thursday.

"We are trying hard to make progress and remain fully engaged to achieve a comprehensive solution by the November 24 deadline."

- More time on the clock -

In months of discussions since an interim agreement struck last November took effect in January, some progress has been made.

This includes possible changes in the design of an unfinished reactor at Arak so that it produces less weapons-grade plutonium, enhanced UN inspections, and alterations to the fortified Fordo facility.

The main bone of contention 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 of sanctions relief, the timeframe that an accord would cover, and a stymied UN probe into past suspect "military dimensions" of Iran's activities.

"Everyone has been working incredibly hard.... These are incredibly complex negotiations, the detail is extraordinary," the senior US official said.

"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."

Zarif told online news site al-Monitor that the gaps were "narrowing, but we still have a long way to go."

"It is reconcilable, provided everyone makes the tough decisions," he said.

Many analysts have begun to believe that the November deadline might be extended, as happened with an earlier target date of July 20, maybe locking in measures related to Arak and Fordo.

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

Zarif himself said Wednesday "more time might be needed" and President Hassan Rouhani said Monday that if the parties are "not able to solve all the problems, the two camps will find a solution".

Echoing Lavrov, Russia's lead negotiator Sergei Ryabkov, who held Thursday bilateral talks with the Iranians, said that "the result is more important than the date".

But quoted by RIA Novosti, he added that with a further extension, "the risk of total failure also increases".

Zarif and the Iranian delegation left Vienna on Thursday afternoon although the six powers remained for talks among themselves.

Zarif said he would see Kerry again in "three or four weeks" -- leaving very little time until November 24 -- following expert-level negotiations next week.