The deadline for a nuclear deal with Iran has been extended to the end of June after talks in Vienna failed to reach a comprehensive agreement.
US Secretary of State John Kerry said that the talks had been “tough”, but “substantial progress” had been made. The parties will reconvene in December.
Iranian leader Hassan Rouhani said most gaps had “narrowed” during the talks.
Six world powers want Iran to curb its nuclear programme in return for the lifting of sanctions.
Tehran says it is not seeking nuclear weapons, but wants atomic energy.
The six countries – the US, UK, Russia, China, France and Germany – have been in negotiations with Iran to finalise a preliminary deal reached last year in Geneva.
They have now agreed to extend discussions, with the aim of reaching a high-level political agreement by 1 March, and confirming the full technical details of the agreement by 1 July.
Iran would be allowed to continue accessing $700m (£450m) a month in frozen assets during that period.
Speaking after the Vienna talks had ended, Mr Kerry said: "We are certainly not going to sit at the negotiating table forever.
“But given how far we have come over the past year, particularly in the past few days, this is certainly not the time to get up and walk away.”
