Deadline day for Iran nuclear deal
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.
"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.
Intense negotiation
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.
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.
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.
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.
Credit: The BBC
