Iran and world powers met in Muscat on Tuesday amid growing signs that a long-bargained deal on Tehran's nuclear programme will not be struck by a November 24 deadline.
The one-day meeting comes after lengthy discussions between US Secretary of State John Kerry and Iran's Foreign Minister Mohammad Javad Zarif broke off in the Gulf sultanate late Monday with no signs of progress.
Tuesday's talks between Iran's nuclear negotiating team and officials from the P5+1 -- -- the United States, France, Britain, Russia and China, plus Germany -- started around 11:45 am (0745 GMT).
The discussions, chaired by former EU foreign policy chief Catherine Ashton, were scheduled to brief the P5+1 members on Kerry and Zarif's talks. Iranian and US officials indicated Monday that big obstacles stand in the way of a final agreement.
The West still appears unconvinced of Iran's assurances that it has not sought and will not attempt to develop an atomic bomb, while Tehran wants clearer undertakings on when sanctions will be lifted, as well as fewer curbs on its nuclear activities.
The US State Department said the head-to-head meetings -- more than 10 hours across two days -- proved "tough, direct and serious", adding that "there is still time" for progress.
Iran's deputy foreign minister Abbas Araqchi, quoted by state media, was more candid.
"To reach a result by November 24 is very difficult but we do not despair," he said.
A US official said Kerry's top two negotiators, Under Secretary of State Wendy Sherman and Bill Burns, a former under secretary, would attend the P5+1 briefing which will also include technical experts on nuclear policy and sanctions.
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- Deal with Russia on reactors -
Kerry, now in Beijing, will speak to President Barack Obama as well as White House national security adviser Susan Rice about the talks with Zarif.
In Muscat, the Iranian delegation held bilateral talks with Russia and China, with similar discussions with Britain, France and Germany scheduled for later in the day.
As the talks in Oman got under way, Iran signalled its intent to pursue bigger atomic plans, announcing a deal that will see Russia build two new nuclear reactors at the Bushehr plant in southern Iran, taking its total number of reactors to eight.
The P5+1 wants Iran to reduce the scope of its nuclear activities -- in exchange for an easing of punitive economic sanctions imposed since 2012 which have hobbled Iran's economy.
Iran says its nuclear programme aims to produce atomic energy to reduce the country's reliance on fossil fuels, but the West and Israel suspect the fuel could be enriched to produce a bomb.
The Iranian delegation is under domestic pressure to deliver a quick and total lifting of US, UN and European sanctions under a deal. Obama, however, said in a television interview on Sunday that sanctions would only be "slowly reduced".
The key sticking points are thought to be the number and type of uranium-enriching centrifuges Iran should be allowed to keep spinning, the process for relieving sanctions and the duration of the final deal. Iran has said five years but world powers have suggested at least double that.
With no signs of a conclusive agreement being near, there are expectations the interim deal implemented in January will be extended. This has already happened 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, with the US diplomat saying "there is no linkage whatsoever" with the nuclear talks.