Iran nuclear talks: Doubts over deal as deadline looms
Doubts are growing at talks in Vienna that a deal on Iran's controversial nuclear programme will be struck before a Monday deadline.
Both the US and Germany said the sides were working to close "big gaps", with some suggestions that the deadline could be extended.
Six world powers want Iran to curb its nuclear programme in return for the lifting of United Nations sanctions.
Iran rejects claims that it is seeking to build nuclear weapons.
It says its programme is purely peaceful for energy purposes.
Representatives of the so-called P5+1 group - Britain, China, France, Russia, the US plus Germany - are taking part in the negotiations with Iran in the Austrian capital.
On Saturday, US Secretary of State John Kerry said: "We're working hard. We hope we're making careful progress, but we have big gaps... which we're working to close."
He spoke after a meeting with Iranian Foreign Minister Mohammad Javad Zarif - their fourth in three days.
German Foreign Minister Frank-Walter Steinmeier said progress was still possible if Iran wanted it. His Russian counterpart Sergei Lavrov is travelling to Vienna to join the talks later on Sunday, Russian media report.
Uranium enrichment: Western states want to reduce Iran's capacity in order to prevent it acquiring weapons-grade material but Tehran is set on expanding it nearly 20-fold in the coming years
Sanctions reduction: Iran wants sanctions lifted immediately but Western states want to stagger their removal to ensure Tehran abides by its commitments
Bomb technology: Iran has failed to explain explosives tests and other activity that could be linked to a nuclear weapons programme and has denied international nuclear inspectors access to its Parchin military site
The BBC's Bethany Bell in Vienna says it is not clear whether a comprehensive deal can be reached by Monday's deadline.
Diplomats at the talks have suggested that more time may be needed: Iran's Isna news agency quotes an unnamed member of the Iranian delegation as saying a deal was impossible by Monday and that an extension was "on the table".
AFP news agency quotes an Iranian source as saying Tehran was open to extending the negotiations by up to a year.
A US official said the aim remained getting a deal by Monday night, "but we are discussing both internally and with our partners a range of options", AFP reported.
The New York Times reported that a key US concern was to prevent Iran from producing a bomb at undetected nuclear locations - referred to as a "sneakout".
The newspaper quoted one participant as saying that the goal was to "make as airtight as possible" the language that would allow highly intrusive inspections.
Meanwhile a correspondent for the newspaper in Tehran reported on Twitter that authorities had allowed a rare protest by hardliners against a deal.
The demonstrators accused President Hassan Rouhani of giving in to Western pressure.
The P5+1 group and Iran reached an interim agreement a year ago. But the two sides failed to reach a lasting deal by July, as initially agreed, and extended the deadline until 24 November.
Iran has been resisting efforts to scale back its nuclear programme for nearly a decade.
Nuclear talks timeline
- January 2012: IAEA confirms Iran is enriching uranium to levels of up to 20%, an important step towards making it weapons-grade
- June 2013: Reformist-backed cleric Hassan Rouhani wins presidential election, raising hopes for deadlocked nuclear talks
- November 2013: Iran and world powers reach interim deal to curb programme; deadline for final agreement set for July 2014
- January 2014: Some Western sanctions eased as Iran uranium enrichment remains at 5%
- July 2014: Deadline extended until November 2014