GENEVA (AFP) – US Secretary of State John Kerry was set to arrive in Geneva Sunday for renewed talks with his Iranian counterpart on Tehran's nuclear program, 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.

Iran denies its nuclear program has military objectives.

Kerry had been set to arrive in the Swiss city Sunday morning for two days of talks with Iranian Foreign Minister Mohammad Javad Zarif, but was delayed in London and by midday had yet to take off from the British capital.

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.

Clock ticking  

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.

But Kerry warned in London Saturday that "there are still significant gaps, there is still a distance to travel."

"President (Barack) Obama has no inclination whatsoever to extend these talks beyond the period that has been set out," Kerry said.

He said the P5+1 was "unified" in its views of what was needed to reach a deal, adding that the Geneva talks would help show "whether or not Iran is able to match its words about its willingness to show that its program is fully peaceful."

As a sign that efforts were intensifying, US Energy Secretary Ernest Moniz arrived in Geneva Saturday to take part in the talks for the first time.

Ali Akbar Salehi, the director of the Iranian Atomic Energy Organization, was also participating in the negotiations.

Observers said Moniz and Salehi's participation was a promising sign that a deal could be within reach.

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.

Another deadline delay unlikely

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 Nov. 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," Kelsey Davenport, head of the Nonproliferation Policy Arms Control Association in Washington, told AFP.

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."