Foreign ministers trying to reach an agreement on Iran's nuclear programme are considering extending negotiations, as Monday's deadline for a deal looms.
The diplomats still report "serious gaps" at the talks in Vienna, and may look to agreeing an interim framework.
The six nations - the US, UK, Russia, China, Germany and France - want Iran to curb its nuclear programme in return for the lifting of UN sanctions.
Iran says it is not seeking nuclear weapons, but wants atomic energy.
It says its nuclear programme is solely designed to provide peaceful, civilian projects.
On Sunday evening, the so-called P5+1 group and Iranian officials said they were discussing the possibility of extending the negotiations, as a comprehensive agreement was still far away.
Analysis: Jeremy Bowen, BBC Middle East editor, Vienna
The price of failure is potentially serious. These talks are the culmination of a lot of work since a temporary agreement in Geneva a year ago. That deal stopped what appeared to be an inexorable slide to war. Israel was threatening to attack Iran's nuclear facilities.
Success would do much to lift Iran's isolation - which would change the strategic balance in the Middle East. That intrigues some Western diplomats. It horrifies Saudi Arabia and Israel.
A year ago all sides in the talks saw an opportunity to deal with an issue that could potentially cause another catastrophic war. That will make them very reluctant to leave Vienna empty-handed - even if that means they will have to do more work on the details next year.
Time is limited though. Hardliners in Tehran and Washington DC will try to sabotage any agreement. Both see no reason to dilute their mutual suspicion.
Significant differences reportedly remain over the scope of Iran's uranium enrichment programme and the timing of sanctions relief.
A senior US state department official told reporters: "Our focus remains on taking steps forward toward an agreement, but it is only natural that just over 24 hours from the deadline we are discussing a range of options... an extension is one of those options."
Earlier, an Iranian news agency quoted an unnamed member of the Iranian delegation as saying a full deal was "impossible" by Monday.
However, Iranian sources have told BBC Persian's Kasra Naji that a "political agreement" could be reached before the deadline.
Diplomats have not managed to come close to a broad and comprehensive agreement, but are thought to be agreeing on a document which at best might mention parameters of a future agreement, he says.
Representatives of the P5+1 - the five permanent UN Security Council members plus Germany - reached an interim deal with Iran, known as the Geneva Accord, last year.
Under the agreement, Iran curbed some of its uranium enrichment in return for sanctions relief.
However, the two sides failed to reach a lasting deal by July, as initially agreed, and extended the deadline until 24 November.
US President Barack Obama told broadcaster ABC that a deal could "perhaps begin a long process in which the relationship... between Iran and the world, and the region, begins to change".
However, he acknowledged that "significant" gaps between the two sides remained, and that Iranian President Hassan Rouhani had "to deal with his politics at home".
"He's not the ultimate decider inside of Iran, the Supreme Leader [Ayatollah Ali Khamenei] is."
Earlier on Sunday, in Tehran, officials approved a rare protest by hardliners who criticised government negotiators.
The demonstrators accused President Rouhani of giving in to Western pressure.
US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have held several rounds of talks in recent days in an attempt to break the deadlock.
On Sunday, Mr Kerry also met Saudi Foreign Minister Saud al-Faisal.
Saudi Arabia is not a party of the P5+1 talks, but is concerned about Iran's influence in the region.
Chinese Foreign Minister Wang Yi is expected in Vienna early on Monday.
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