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 programme 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.
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.
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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."
Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation, 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 signalled a breakthrough, saying 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.