Iran's foreign minister warned Tuesday of "numerous" gaps in troubled nuclear talks with world powers but both he and US counterpart John Kerry, due in Vienna Wednesday, insisted a deal remains possible by a November 24 deadline.

Speaking early Tuesday as he arrived in Vienna for talks with US and EU officials, Foreign Minister Mohammad Javad Zarif said there was "general agreement" but that "numerous questions still need to be resolved".

Quoted by Iranian media he added: "We will make every effort to make progress in the coming days."

US Secretary of State Kerry, speaking in Paris a day before he was due to join Zarif in Vienna, said that although hard work remained to be done a deal was achievable.

"I don't believe it's out of reach, but we have some tough issues to resolve," Kerry told reporters in the French capital after talks with Russian Foreign Minister Sergei Lavrov.

He refused to be drawn on whether -- as speculated by many experts -- Iran and the six powers might push back the deadline in order to gain more time to bridge the differences.

"We need to continue to have some serious discussions, which we will, and we'll see where we are. And I just think I'll let the negotiation process speak for itself at this point in time," Kerry said.

"I don't think anything is served by a lot of speculation."

But Russia's Lavrov, whose country together with the US, China, Britain, France and Germany forms the P5+1 group, said at a separate news conference in Paris that the November 24 deadline was not "sacred".

"I am sure that a compromise is possible. But I cannot guarantee that this can absolutely be done by November 24. This is not a sacred date," Lavrov said, according to Interfax.

"We aspire to get a result by that date but I am convinced by the principle that it is not artificially-set deadlines but the essence of the deal, the quality of the deal (that counts)," Lavrov said.

Speculation about a possible extension has been stoked by comments from Iranian officials, with Araqchi saying on Friday such a move was "possible" and even President Hassan Rouhani not ruling it out.

"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 Monday.

- Grand bargain -

Negotiating intensively for months, Washington and the P5+1 -- the five permanent UN Security Council members, plus Germany -- want Iran to reduce the scope of its atomic activities in return for relief from painful sanctions.

After more than a decade of rising tensions, this would ease long-held fears -- rejected by Iran -- that the Islamic republic wants to develop nuclear weapons under the guise of its civilian atomic activities.

In November 2013, Iran and the P5+1 struck an interim deal and gave themselves initially until July 20 to reach a lasting accord. They missed that deadline and agreed to push it back to November 24.

The main problem issue remains the central question of Iran's future capacity to enrich uranium, a process that can produce fuel for reactors but also, at high purities, the core of a nuclear bomb.

- Mini-round -

The Vienna talks on Tuesday, following discussions in New York last month, involved Wendy Sherman, Washington's lead negotiator, her Iranian counterpart Abbas Araqchi and the powers' chief negotiator, EU foreign policy head Catherine Ashton.

Russia's chief negotiator, Sergei Ryabkov, said in Moscow that a "mini-round" involving all Iran and the six powers would take place in Vienna on Thursday after Kerry and Zarif's discussions, Interfax reported.

A spokesman for Ashton confirmed that to reporters in Vienna, saying the all-in meeting would see Zarif meeting P5+1 political directors. The State Department said it would last one day only.

Analyst Kelsey Davenport from the Arms Control Association told AFP that this week's talks were "critical" and that a deal by November 24 was "still possible if both sides... are willing to be flexible and creative".