Pressure is mounting on world powers and Iran to reach a deal on its nuclear programme and avoid the issue becoming bogged down in the US Congress.
If a final agreement is not reached by Friday, Congress will double the time it takes to review an accord. That will delay the lifting of US sanctions.
The pace at which sanctions are lifted is a major issue at talks in Vienna.
World powers and Iran are still locked negotiations, despite missing a self-imposed 30 June deadline.
The so-called P5+1 (US, UN, Russia, UK, France and Germany) and Iran are still believed to differ in three key areas – international inspections of Iran’s non-nuclear sites, sanctions, and how Iran’s compliance will be verified.
Iran also wants a UN Security Council arms embargo to be scrapped – something the US has ruled out.
World powers want to be satisfied that Iran is neither trying to develop a nuclear bomb, nor has the capability to do so in under at least a year. Iran insists its nuclear programme is for purely peaceful purposes.
If a deal is reached in Vienna, under US law Congress has 30 days in which to decide whether to accept or reject it, so long as it receives the text of the agreement by midnight Washington DC time (05:00 GMT on Friday).
Failing that, the review period will be extended to 60 days. During this time, US sanctions will remain in place.
If Congress rejects the deal, President Barack Obama can use his power of veto, but Congress can still overturn this. An extended review could help opposition to a deal build in the Republican-controlled legislature.
Under an interim accord, Iran and the P5+1 agreed that crippling sanctions would be eased in return for curbs on Iran’s nuclear programme.
Both sides at the Vienna talks said they hoped for progress on Thursday, though there has been little concrete sign of a breakthrough.
“Hopefully today is the last day,” Iranian nuclear chief Ali Akbar Salehi said, as he headed into a meeting with US Energy Secretary Ernest Moniz.
Mr Moniz said they were “going to resolve the last issues, if we can”.