Diplomats from six major nations and Iran are meeting in Vienna, and face a 23:00 GMT deadline to resolve a dispute that has lasted 12 years.
They said earlier that major differences remained, suggesting they could now aim to extend the talks.
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.
"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.
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