Iran and world powers have given themselves four more months to negotiate a historic nuclear deal after failing to close major gaps in marathon talks in Vienna.
"While we have made tangible progress on some of the issues and have worked together on a text (for a deal) ... there are still significant gaps on some core issues," lead negotiator and EU foreign policy chief Catherine Ashton said on Saturday, adding the talks would continue until November 24.
As part of the deal, the US said it would unblock some $US2.8 billion ($A3.03 billion) in frozen funds, in return for Iran converting a quarter of its 20 per cent enriched uranium stocks - which can be used to make a bomb - into fuel.
American officials will leave Vienna over the weekend with the aim of resuming talks, perhaps at expert level, in August. The UN general assembly in September is also expected to provide a "fulcrum" for the next phase of negotiations, one US administration official said.
In a statement repeated in Farsi by Iranian Foreign Minister Mohammad Javad Zarif, Ashton said the parties would "reconvene in the coming weeks ... with the clear determination to reach agreement ... at the earliest possible moment".
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In November last year, Iran and the five permanent members of the UN Security Council plus Germany agreed on an interim deal under which the Islamic republic froze certain nuclear activities for six months in return for some sanctions relief.
This expires on July 20, but the parties had given themselves the option to push back this deadline if they failed during the six months to transform the interim deal into a lasting accord.
US Secretary of State John Kerry, who this week spent two days in Vienna trying to broker a breakthrough, said on Friday the "short extension" was "warranted by the progress we've made and the path forward we can envision.
"To turn our back prematurely on diplomatic efforts when significant progress has been made would deny ourselves the ability to achieve our objectives peacefully," he said.
The deal would ease fears that despite its denials Iran is seeking to develop nuclear weapons after a decade of atomic expansion.
The deal under negotiation is highly ambitious and fiendishly complex.
The six powers want Iran to dramatically reduce its nuclear program for a lengthy period of time and agree to more intrusive UN inspections.
This would expand the time needed for Tehran to develop a nuclear weapon, while giving the world ample warning of any such "breakout" push.
The two sides are believed to have narrowed their positions in recent weeks on a few issues such as the Arak reactor, which could give Iran weapons-grade plutonium, and enhanced inspections.
But they remain far apart on the key issue of Iran's capacities to enrich uranium, a process which can produce fuel for reactors but also the core of a nuclear bomb.
