US Secretary of State John Kerry (left) meeting Iran's Mohammad Javad Zarif on Sunday
Negotiators in Vienna, Austria, are set to make a final push to try to reach a deal on Iran's nuclear programme ahead of a midnight (23:00 GMT) deadline.
They said earlier that "serious gaps" remained, suggesting that an extension could be agreed instead.
World powers - the US, UK, Russia, China, France and Germany - want Iran to curb its nuclear programme in return for the lifting of UN sanctions.
Tehran says it is not seeking nuclear weapons, but wants atomic energy.
"At the moment, we are focused on the last push... to try and get this across the line," British Foreign Secretary Philip Hammond said.
UK foreign secretary Philip Hammond: "We're still quite a long way apart"
"Of course, if we're not able to do that, we will then look at where we're going from there."
A senior US state department official told reporters: "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."
The delegates could also set out a framework of a deal that would be finalised later.
Iranian President Hassan Rouhani has said that "if the West does not make excessive demands, an agreement is within reach", according to Iran's Resalat newspaper.
Chinese Foreign Minister Wang Yi is expected to arrive in Vienna, meaning the foreign ministers of all of the so-called P5+1 group will be in attendance.
Analysis: Jeremy Bowen, BBC Middle East editor, Vienna
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 in Geneva 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.
When US Secretary of State John Kerry met his Iranian counterpart Mohammad Javad Zarif on Sunday - their sixth encounter in four days of talks - an extension was discussed, according to an unnamed Iranian source who spoke to AFP news agency.
The proposed agreement hinges on the rate at which the sanctions against Iran would be lifted if Tehran agreed to highly specific limits on the amount of uranium it could enrich.
It has been a gruelling weekend of negotiation for the West's top diplomats
At the heart of the process have been the six rounds of talks between John Kerry (left) and Iran's Mohammad Javad Zarif, seen here with the EU's Catherine Ashton on Saturday
The cameras are rarely far away for these much-anticipated talks: here Mr Kerry makes a phone call from a Vienna park to Canadian Foreign Minister John Baird on Saturday
Highly enriched uranium can be used to make a nuclear bomb, but uranium enriched to lower levels can be used for energy purposes.
Under the terms of international treaties, countries have the right to develop nuclear energy - which Iran insists is its only aim.
President Obama told ABC a deal could mark the start of a "long process"
Under an agreement reached in Geneva last year, Iran was to curb 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 acknowledged on Sunday that "significant" gaps between the two sides remained, and that President 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," Mr Obama told US broadcaster ABC.
Students in Tehran held a protest in support of Iran's nuclear programme on Sunday
Earlier on Sunday Iranian sources told BBC Persian in Vienna that a "political agreement" might be reached before the deadline.
However, an extension of six months to a year "would not fly" with the other parties, Arms Control Association analyst Kelsey Davenport told AFP.
Any extension would "have to be very short because there are too many hardliners, particularly in Washington and Tehran, that want to sabotage this deal", she added.
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
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