The head of the International Atomic Energy Agency, Mohamed ElBaradei, has given Iran and three world powers the text of a draft deal aimed at reducing concerns about Iran's nuclear programme.
The UN agency wants to persuade Iran to allow most of its uranium to be shipped abroad for further enrichment before being returned for use in a civilian research reactor.
Iran maintains its nuclear ambitions are peaceful; but its critics are not convinced.
The draft document came at the end of talks at the IAEA headquarters in Vienna.
Dr ElBaradei said he hoped all parties would respond positively by Friday:
The BBC reports the proposal is thought to involve Iran handing over three quarters of its stockpile of low enriched uranium to Moscow.