World powers and Iran geared up Monday for fresh nuclear negotiations, with Russian President Vladimir Putin telling Iranian counterpart Hassan Rouhani he was upbeat about prospects for an accord, even though the US downplayed hopes of an imminent deal.

Speaking by phone two days before the talks resume on Wednesday in Geneva, Putin "stressed that a real chance has now emerged for finding a solution to this longstanding problem," the Kremlin said.

The comments came a day after French President Francois Hollande laid out in Israel the "essential" steps that Tehran must agree with the United States, China, Russia, Britain, France and Germany, known collectively as the P5+1 group.

These steps include halting enrichment of uranium to 20-percent purity, reducing enriched uranium stockpiles, and stopping construction of a new reactor at Arak, which could produce plutonium.

But Hollande stressed in Jerusalem on Monday that "we will maintain the sanctions as long as we are not certain that Iran has definitively renounced its military (nuclear) programme."

And it remains to be seen whether the small and reversible sanctions relief that the P5+1 is offering in return will be enough to persuade Tehran to play ball.

After US officials last week said a deal was "quite possible," US Secretary of State John Kerry was more cautious on Monday saying: "I have no specific expectations with respect to the negotiations in Geneva, except that we will continue to negotiate in good faith."

"We will try to get a first step agreement and hope that Iran will understand the importance of coming there prepared to create a document that can prove to the world that this is a peaceful programme," Kerry said.

Israel and the West have long accused Iran of seeking a nuclear weapons capability, but Tehran insists its controversial uranium enrichment programme is for entirely peaceful purposes.

Years of US and EU sanctions have more than halved Iran's oil sales, sent the currency plummeting and inflation soaring.

Newly elected Iranian President Rouhani, who has raised hopes for an end to the decade-old standoff, told Putin however that "excessive demands could complicate the process towards a win-win agreement," an Iranian government website said.

One potential sticking point in the negotiations is Iran's demand that the powers recognise it has a "right" to enrich uranium.

"No agreement will be reached without securing the rights of the Iranian nation," the lead Iranian negotiator Abbas Araqchi said Sunday, predicting "difficult" talks.

Obama to meet senators

Even though it would only be a "first phase" initial deal, an accord in Geneva would be a major breakthrough after a decade of rising tensions and failed diplomatic initiatives.

But watching with a sceptical eye will be hardliners in both the United States and Iran, as well as Israel.

US President Barack Obama on Tuesday will personally urge American lawmakers to hold off on further sanctions, to give the diplomacy being led by the so-called P5+1 group time to work.

Obama will meet with leading senators on the eve of the talks to press his view "that it's the right thing to do for Congress to pause so that we can test whether or not the Iranians are serious about resolving this issue diplomatically," White House spokesman Jay Carney said.

And in the high-stakes talks, Rouhani, who only took up office in August, also risks losing the backing of supreme leader Ayatollah Ali Khamenei if the relative moderate's "charm offensive" fails to bear fruit soon.

"If Rouhani is not getting anywhere, the conservatives are going to make a strong comeback," Trita Parsi, author and president of the National Iranian American Council, told AFP.

The toughest to satisfy could be Israel, which sees its very existence threatened by a nuclear-armed Iran allied with Hezbollah and Syrian President Bashar al-Assad.

Three days of talks in Geneva failed earlier this month to clinch a deal even though Kerry and all the other five foreign P5+1 ministers flew to the Swiss city hoping to narrow the gaps.

Kerry has not yet decided whether he will again join the negotiation this weekend, but remains "open" to the idea, his spokeswoman said Monday.