Marathon talks between Iran and world powers in Vienna ended yesterday after negotiators gave themselves four more months to try and bridge major gaps and strike a historic nuclear deal.
New rounds of talks were expected in the coming weeks, with the date and place yet to be decided, diplomats said.
"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 told journalists in the early hours of Saturday.
The talks will now continue until November 24, she added.
Under the terms of the extension, the United States said it would unblock some $2.8 billion (2.1 billion euros) in frozen funds, in return for Iran converting a quarter of its 20-percent enriched uranium stocks -- which can be used to make a bomb -- into fuel.
American officials left Vienna 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".
Last November, Iran and the five permanent members of the UN Security Council plus Germany agreed an interim deal under which the Islamic republic froze certain nuclear activities for six months in return for some sanctions relief.
But they gave themselves the option of pushing back the July 20 deadline if they failed to transform the interim deal into a lasting accord in that time.
US Secretary of State John Kerry, who this week spent two days in Vienna trying to broker a breakthrough, said 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," Kerry said.