Iran nuclear crisis: Oman hosts talks as deadline nears
High-level talks have taken place in Oman ahead of the 24 November deadline for a comprehensive deal on Iran's nuclear programme.
US Secretary of State John Kerry met his Iranian counterpart Mohammad Javad Zarif and EU envoy Catherine Ashton.
But the talks ended with no immediate breakthrough after running into an unscheduled second day. One US official described them as tough and direct.
Iran denies claims by world powers that it is trying to make a nuclear weapon.
It insists its uranium enrichment programme is purely for peaceful purposes.
Speaking as the talks began on Sunday, US President Barack Obama said "big gaps" remained in negotiations concerning uranium enrichment and sanctions relief.
He admitted that a deal may not be made before the 24 November deadline.
The meeting came a year after Iran and six world powers - the US, Britain, France, Germany, Russia and China - agreed an interim deal to limit Iran's nuclear activities in exchange for some relief from Western sanctions.
Iran promised to further co-operate with the International Atomic Energy Agency (IAEA) following the election of President Hassan Rouhani last year in return for an easing of the sanctions.
A recent confidential IAEA report seen by the BBC said Iran was failing to answer questions about suspected covert activity at some of its nuclear facilities.
It said that although Iran agreed in May to provide information on two out of about a dozen areas of suspicion by August, it is yet to give answers.
The BBC's Barbara Plett Usher in the Omani capital Muscat says both sides recognise there is a historic opportunity, and have expressed concern about the consequences of failing to reach a deal by 24 November.
Mr Kerry has insisted that the negotiations are not linked to possible co-operation with Iran in the regional fight against Islamic State militants in Syria and Iraq.
Recent reports in the US media described a letter from President Barack Obama to Iran's Supreme Leader Ayatollah Ali Khamenei suggesting a nuclear deal could benefit their common interest in fighting the group.