U.S. Secretary of State John Kerry, left, European Union High Representative Catherine Ashton, center, and Iranian Foreign Minister Mohammad Javad Zarif meet in Muscat, Oman, Monday, Nov. 10, 2014 (Photo: AP)
Top officials from Iran and six world powers meet in Muscat Tuesday as fears grow that a hard-fought deal on Iran's nuclear programme may not be reached by this month's deadline.
The talks come after two days of discussions between US Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif ended with neither man revealing what remains unsolved in the long-running bid for a comprehensive accord.
Each laid out their demands in over 10 hours of private talks that sought to overcome deep differences, allay mutual suspicion, and bring 12 months of diplomatic brinkmanship to the point of a breakthrough.
Asked if they were making progress, as they appeared briefly for photographers, Zarif replied: "We will eventually".
Kerry said: "We are working hard. We are working hard".
On Tuesday the P5+1 powers -- made up of the five permanent UN Security Council members the United States, France, Britain, Russia and China, plus Germany -- will take part in the Muscat talks chaired by former EU foreign affairs chief Catherine Ashton, who will see out the nuclear negotiations until November 24.
The six world powers want Iran to reduce the scope of its atomic activities. In exchange, the West is offering to ease punishing economic sanctions imposed since 2012.
US President Barack Obama said Sunday that a "big gap" remained on how the West can have "verifiable, lock-tight assurances" that Iran cannot obtain a nuclear weapon.
"We may not be able to get there," Obama told CBS News.
Iran denies it is seeking a bomb and says its nuclear programme aims to produce atomic energy to reduce the country's reliance on fossil fuels, requiring a massive increase in its ability to enrich uranium in coming years.
The Iranian delegation is under pressure to deliver a quick and total lifting of US, UN and European sanctions under a final deal. Obama, however, said sanctions would only be "slowly reduced" if Tehran meets its obligations.
The key sticking point is thought to be the number and type of uranium-enriching centrifuges Iran should be allowed to keep spinning in exchange for sanctions relief and rigorous inspections of its nuclear sites.
The duration of a final settlement plan also remains contested, with Iran speaking of five years and world powers suggesting at least double that.
After the talks between Kerry and Zarif ended on Monday, the US State Department said they had proved "tough, direct and serious", while adding "there is still time" for progress.
Iran's deputy foreign minister Abbas Araghchi said "no progress" had been made during the two days.
"We can no longer talk about progress in the negotiations, but we are optimistic that we can reach an accord" before November 24, he said, quoted by Iranian news agency ISNA.
Some analysts have said a deal may already be out of reach.
"A full-fledged agreement is no longer possible before the deadline. What is still achievable is a breakthrough that could justify adding more time to the clock," Ali Vaez, senior Iran analyst at the International Crisis Group, told AFP.
The talks have already been extended once -- when a July 20 deadline was missed. Despite the logjam, neither side has indicated it would walk away from the table.
The meetings in Muscat follow the revelation that Obama reportedly wrote to Iran's supreme leader Ayatollah Ali Khamenei to push for a deal, arguing the Islamic republic and the West have shared regional interests.
This apparent reference to the fight against Islamic State group militants in Syria and Iraq was played down by Kerry, however, with the US diplomat saying "there is no linkage whatsoever" with the nuclear talks.
Domestic politics are also a factor, given the Democrats' loss of the Senate in midterm elections to the Republican party, whose members have consistently bridled at the White House's negotiations with Iran.
If talks go sour in the coming weeks it is thought US Congress may respond with fresh sanctions on Iran.
Obama has the power to veto them, but the prospect of new penalties could push already protracted dealmaking towards being untenable for the Iranian government.
Zarif and President Hassan Rouhani are under fire from lawmakers sceptical of the interim deal -- which came into force in January -- who have also said a bigger, final agreement must be ratified by parliament.