GENEVA (AFP) - There are still "significant gaps" in negotiations over Iran's nuclear programme, US Secretary of State John Kerry said Saturday, warning that President Barack Obama was not prepared to extend them further.
Kerry's comments came in a stopover in London before he heads to Geneva Sunday for two more days of talks with Iranian Foreign Minister Mohammad Javad Zarif.
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 Teheran from developing a nuclear bomb in return for easing economic sanctions.
Iran denies its nuclear programme has military goals.
Negotiators are working against the clock ahead of a March 31 deadline for agreement on the political framework of a deal.
"There are still significant gaps, there is still a distance to travel," Kerry told a press conference at the US Embassy in London.
"President Obama has no inclination whatsoever to extend these talks beyond the period that has been set out."
He added that Obama was "fully prepared to stop these talks" if necessary.
US and Iranian negotiators have been meeting in Geneva since Friday and senior P5+1 negotiators are also set to meet in the Swiss city Sunday in a bid to drive the talks forward.
Kerry also used his London stop to stress the international community was "united" and "in lock step" over the negotiations.
"There is absolutely no divergence whatsoever in what we believe is necessary for Iran to prove that its nuclear programme is going to be peaceful," he said earlier in the day.
US Energy Secretary Ernest Moniz flew in to snow-covered Geneva on Saturday to take part in the talks for the first time, and at Kerry's request.
Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation, was also taking part in the negotiations.
But Kerry played down any suggestion that this meant the talks were on the verge of a breakthrough.
"I would not read into it any indication whatsoever," he said, adding that Moniz was present because of the "technical" nature of the talks.
The two officials were reportedly meeting Saturday afternoon.
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 because two previous deadlines have been missed.
Iranian officials have 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 Teheran can retain.
Under an interim deal reached in November 2013, Iran's stock of fissile material has been diluted from 20 per cent enriched uranium to 5 per cent in exchange for limited sanctions relief.
Experts say such measures pushed back the "breakout capacity" to make an atomic weapon.
Negotiations have been complicated by hardliners both in Iran and the United States, as well as by Israel 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.
Meanwhile, Teheran wants massively to ramp up the number of enrichment centrifuges, saying it wants to make fuel for a fleet of power reactors it has yet to build.