Iran, the United States and the European Union (EU) began an unscheduled second day of talks on Monday over disagreements blocking the resolution of a confrontation over Tehran's nuclear programme, US and Iranian officials said.
With two weeks to a deadline for an overall agreement, Iranian Foreign Minister Mohammad Javad Zarif, US Secretary of State John Kerry and EU envoy Catherine Ashton met in Oman's capital Muscat to tackle a decade-long dispute which has raised the risk of wider conflict in the Middle East.
The discussions aim to put verifiable limits on Iran's uranium enrichment work - and any other potential path to a nuclear weapon - in return for a gradual lifting of sanctions.
US President Barack Obama told CBS television there was still a big gap between Iran and Western powers and said a deal could be out of reach.
Economic sanctions led by the United States have pushed Iran to the table for an agreement on its nuclear programme, Obama said in an interview broadcast on Sunday.
A final step would involve Iran providing "verifiable, lock-tight assurances that they can't develop a nuclear weapon", Obama said, adding: "There's still a big gap. We may not be able to get there."
Western countries suspect Iran has secretly attempted to acquire the means to build nuclear weapons.
Iran says it wants peaceful nuclear energy only, but has refused to curb enrichment capacity and has been hit by damaging US, EU and UN Security Council sanctions as a result.
In the CBS interview, Obama declined to comment on a report that he had sent a letter to Iran's Supreme Leader, Ayatollah Ali Khamenei, about combating the Islamic State militant group.
Obama said the United States and Iran shared a common enemy in Islamic State, but he reiterated the nuclear talks were not connected to the fight against Islamic State, also known as ISIL. "We're not coordinating with Iran on ISIL," he told CBS.
Reaching out
An editorial on Khamenei's website on Sunday made an indirect reference to a letter from Obama, suggesting it was evidence of an unproductive US policy.
It said Obama had written three letters in total to Khamenei -- in 2009, in 2012 and "about a month ago", adding: "In fact, the US has always reached out to Iran when faced with an impasse, and Obama's latest letter is a direct link to foreign policy dead-ends, especially those involving Iran somehow."
The new US Senate, in which opposition Republicans will have a majority following midterm elections last Tuesday, is likely to be sceptical of any Obama deal with Iran.
The toughest outstanding issues are the size of Iran's enrichment programme, the length of any long-term accord and the pace at which sanctions would be phased out, say Western and Iranian diplomats involved in the negotiations.
Washington also wants intensive verification and monitoring measures to ensure Iran is fulfilling its end of the bargain.
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