US Secretary of State John Kerry was set to arrive in Geneva Sunday for renewed talks with his Iranian counterpart on Tehran's nuclear programme, after warning "significant gaps" remain ahead of a key deadline.
World powers are trying to strike a deal with Iran that would prevent Tehran from developing a nuclear bomb in return for an easing of punishing international economic sanctions.
Kerry is set to arrive in the Swiss city Sunday morning for two days of talks with Iranian Foreign Minister Mohammad Javad Zarif, whose country denies its nuclear programme has military objectives.
"There are still significant gaps, there is still a distance to travel," Kerry said in London on Saturday.
There is a heightened sense of urgency as the clock ticks down towards a March 31 deadline to agree on a political framework for the deal.
"President (Barack) Obama has no inclination whatsoever to extend these talks beyond the period that has been set out," Kerry said.
US and Iranian diplomats have been meeting in Geneva since Friday, and senior negotiators from the so-called P5+1 group of Britain, China, France, Russia, the United States and Germany were also expected to meet on Sunday to help drive the talks forward.
Kerry stressed Saturday that there was "absolutely no divergence whatsoever in what we (the P5+1) believe is necessary for Iran to prove that its nuclear program is going to be peaceful."
As a sign that efforts were intensifying, US Energy Secretary Ernest Moniz flew in to snow-covered Geneva Saturday to take part in the talks for the first time.
Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation, was also participating in the negotiations.
The two officials and their delegations spent five hours at the negotiating table Saturday, Iranian media reported.
Observers said Moniz and Salehi's participation was a promising sign that a deal could be within reach.
Kelsey Davenport, head of the Nonproliferation Policy Arms Control Association in Washington, pointed out in an email to AFP that Moniz with his technical expertise would "be a key validator when a deal is concluded."
Salehi, who plays a similar role, would "likely be instrumental in selling the agreement in Tehran," he added.
But Kerry played down any suggestion that their participation meant the talks were on the verge of a breakthrough, adding that Moniz was present because of the "technical" nature of the discussions.
While the political aspects of the deal must be nailed down by the end of next month, the deadline for signing the full agreement is June 30 -- a cut-off point that looms all the larger after two previous deadlines were missed.
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 Israel lobbying against a deal.
Israeli Prime Minister Benjamin Netanyahu is controversially planning to address the US Congress on the issue next month in a move critics say is aimed at torpedoing the 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.
The push instead would reportedly be towards strictly limiting the amount of enriched uranium Iran would be allowed to stockpile in the country.
Observers stressed the urgency of reaching an agreement and warned that a failure to do so would have dire consequences.
"Another extension is extremely unlikely," Davenport said.
If no agreement is reached, he warned, both sides will likely head "back down the path of escalation, with Washington increasing sanctions and Iran ramping up its nuclear program."