The world powers negotiating with Iran over its nuclear programme stand united, US Secretary of State John Kerry said Saturday, a day before resuming talks with his Iranian counterpart.

"There is absolutely no divergence whatsoever in what we believe is necessary for Iran to prove that its nuclear program is going to be peaceful," Kerry said in London before heading to Geneva Sunday for talks with Iranian Foreign Minister Mohammad Javad Zarif.

"The P5+1 remains united on the subject of Iran," he added.

Iran and the P5+1 group of Britain, China, France, Russia, the United States and Germany are trying to strike a deal that would prevent Tehran from developing a nuclear bomb in return for an easing of punishing economic sanctions. Iran denies its nuclear programme has military objectives.

Zarif and Kerry, who have met repeatedly on the issue in recent weeks, will hold two days of discussions in Geneva starting Sunday.

The meeting comes amid a heightened sense of urgency in the negotiations as the clock ticks down to a March 31 deadline for agreement on the political framework of a comprehensive deal.

US and Iranian negotiators have been meeting in Geneva since Friday, and senior P5+1 negotiators were also set to meet in the Swiss city Sunday in a bid to drive the talks forward, the European Union said.

- Top technical negotiators involved -

As a sign the efforts were stepping up a notch, US Energy Secretary Ernest Moniz flew in to snow-covered Geneva Saturday to take part in the talks for the first time, and at Kerry's request, the energy department said.

Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation, was also taking part in the negotiations.

A senior US State Department official warned, however, that Moniz and Salehi's participation did not necessarily indicate the talks were on the verge of a breakthrough, but said it indicated "the seriousness with which they are taking the technical aspects."

The two officials were reportedly meeting Saturday afternoon to discuss the technical aspects of the agreement under discussion.

Salehi arrived Saturday morning with Zarif and Hossein Fereydoun, the brother and special aid to Iranian President Hassan Rohani, to help coordinate the talks, Iranian media reported.

While the political aspects of the deal must be nailed down by the end of next month, the full agreement must be signed by June 30 -- a cut-off point that looms all the larger after two previous deadlines have been missed.

- No two-step deal -

Iranian officials have meanwhile voiced unhappiness with separating the political and technical aspects of an agreement.

"We won't have a two-stage deal," deputy foreign minister Abbas Araghchi said.

"After a year of negotiations, we must tackle the details and all the more so as we want to have both the general framework and the details in the final agreement."

A key stumbling block in any final deal is thought to be the amount of uranium Iran would be allowed to enrich, and the number and type of centrifuges Tehran can retain.

Under an interim deal reached in November 2013, Iran's stock of fissile material has been diluted from 20 percent enriched uranium to five percent in exchange for limited sanctions relief.

Experts say such measures pushed back the "breakout capacity" to make an atomic weapon, which Iran denies pursuing in the first place.

Negotiations have been complicated by hardliners both in Iran and the United States, as well as by Israeli lobbying against a deal.

Nearly two dozen US House Democrats on Thursday urged the postponement of Israeli Prime Minister Benjamin Netanyahu's planned March 3 speech to Congress on the threat of Iran's nuclear programme, warning the timing of the controversial address could "undermine" negotiations.

Israeli officials have also allegedly leaked purported details from the talks showing the US was moving towards softening its demands on how many of Iran's some 20,000 centrifuges it can retain.

Tehran meanwhile wants to massively ramp up the number of enrichment centrifuges -- in order, it says, to make fuel for a fleet of power reactors that it has yet to build.