The United States and Iran held high-level talks in Oman as the deadline for a nuclear deal loomed closer, but President Barack Obama warned there might be no agreement.

US Secretary of State John Kerry met Iran's Foreign Minister Mohammad Javad Zarif in the Gulf sultanate on Sunday, seeking to resolve key disputes that have left the West's negotiations with the Islamic republic close to deadlock.

The two countries are now facing pressure at home over the talks with Obama reiterating in a CBS News interview screened on Sunday that the two sides were still wide apart.

"Are we going to be able to close this final gap so that [Iran] can re-enter the international community, sanctions can be slowly reduced and we have verifiable, lock-tight assurances that they can't develop a nuclear weapon?" Obama asked.

"There's still a big gap. We may not be able to get there," he said.

Kerry and Zarif had two meetings lasting more than five hours in Muscat with former EU foreign policy chief Catherine Ashton, the lead negotiator in the talks, also present.

No statements were made when the meeting broke off for the day but the talks will resume on Monday, officials said.

The meeting in Muscat follows the revelation that Obama reportedly wrote to Iran's supreme leader Ayatollah Ali Khamenei to push for a deal, arguing that Tehran and the West have shared regional interests.

This apparent reference to the fight against Islamic State and Levant (ISIL) in Syria and Iraq was played down by Kerry, however, with the US diplomat saying in Beijing on Saturday, "there is no linkage whatsoever" with the nuclear talks.

After Kerry and Zarif meet again on Monday, the political directors of the P5+1 powers -  Britain, China, France, Russia and the United States plus Germany - will hold talks the following day, also in Muscat.

The main negotiations then move back to Vienna on November 18 for a final push towards the deadline.

An interim accord expires on November 24 but Iran and world powers have for months been unable to hammer out what a comprehensive, long-term accord would look like.

At issue is the number of uranium-enriching centrifuges Iran should be allowed to keep spinning in exchange for sanctions relief and rigorous inspections at its nuclear sites.

The duration of a final settlement between Iran and the P5+1 group also remains contested.

The West is unconvinced by Tehran's denials that it has never sought a nuclear weapon and wants curbs that would put an atomic bomb forever beyond reach.

Iran, however, insists its nuclear programme is for peaceful, civilian energy production only and wants to vastly enhance its uranium enrichment capabilities for this purpose. The country has vowed to do nothing that would roll back its nuclear activities.

Source: Agencies