Time is running out for world powers to strike a deal with Iran to cut UN sanctions in return for action to scale back its nuclear programme.
With major differences remaining, it appears increasingly likely that the talks in Vienna will be adjourned until December, diplomatic sources say.
Iran and the six powers it is negotiating with had set themselves a deadline of 23:00 GMT to reach a deal.
Tehran says it is not seeking nuclear weapons, but wants atomic energy.
The US, UK, Russia, China and France, plus Germany, are trying with Iran to finalise a preliminary deal reached last year in Geneva.
High-level meetings are continuing between Iranian Foreign Minister Javad Zarif, US Secretary of State John Kerry and the other five nations’ foreign ministers.
Over the past few months the talks have reduced fears of a new Middle Eastern war and the ministers will be reluctant to leave Vienna without a way forward, the BBC’s Bethany Bell reports from Vienna.
“Given progress made this weekend, talks [are] headed to likely extension with experts and negotiating teams reconvening in December at a yet to be determined location,” an unnamed Western diplomat told AFP news agency.
A source quoted by Reuters said a new round of talks could be held in the Gulf state of Oman.
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 twentyfold 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