The United States and Iran have held high-level talks in Oman ahead of a looming deadline for a nuclear deal, but President Barack Obama warned that a "big gap" remained.
US Secretary of State John Kerry met Iran's Foreign Minister Mohammad Javad Zarif in the Gulf sultanate on Sunday, with both sides facing political pressure at home over the long-running negotiations which have appeared close to deadlock for months.
Iran and world powers have set November 24 as a deadline to turn an interim agreement into a long-term settlement, but Obama warned it may not be possible.
"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 told CBS News in an interview broadcast on Sunday.
"There's still a big gap. We may not be able to get there."
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Despite the deadline, Iran and the P5+1 group - Britain, China, France, Russia and the United States plus Germany - are far apart on what capabilities Iran's nuclear programme should have.
The West has as yet been unconvinced by Iran's denials that it has never sought a nuclear weapon, while Tehran insists its atomic activities are for peaceful, civilian energy purposes only.
A deal, for the West, aims to put a bomb forever beyond Iran's reach.
Kerry and Zarif met for three hours in Muscat with former EU foreign policy chief Catherine Ashton, the lead negotiator in the talks, also present.
Both sides then broke for lunch and separate private discussions with aides.
No statements were made before the main talks resumed in the evening.
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.
Iran wants "industrial grade enrichment" beyond its current capabilities while the world powers want a reduction.
