Iran and world powers will in May "move to the next phase" of their talks aimed at clinching a definitive nuclear deal, they said in a joint statement after their latest meeting in Vienna.
"We will now move to the next phase in the negotiations in which we will aim to bridge the gaps in all the key areas and work on the concrete elements of a possible comprehensive agreement," said EU foreign policy chief Catherine Ashton who is the chief negotiator for the world powers.
In a statement repeated in Farsi by Iran's foreign minister Mohammad Javad Zarif, Ashton said a "lot of intensive work will be required to overcome the differences which naturally still exist at this stage in the process".
Both described the latest round between Iran and the five permanent members of the UN Security Council plus Germany as "substantive and detailed ... covering all the issues which will need to be part of a comprehensive agreement".
The permanent council members are the United States, Russia, China, Britain and France.
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The fourth and next round will be held in Vienna again from May 13, they said.
A senior US administration official involved in the talks had said last week that the negotiators hoped to make enough progress in this round to begin drafting a deal in May.
In November the two sides reached an interim deal under which Iran froze certain parts of its nuclear activities in return for minor relief from painful Western sanctions.
But Iran has not permanently dismantled any of its nuclear equipment and can fully reactivate its facilities if it wishes when the deal expires on July 20.
Earlier on Wednesday, Iran's supreme leader Ayatollah Ali Khamenei said "Iran's activities in nuclear research and development as well as its nuclear achievements will never be stopped".
Iran's lead negotiator at the talks said late Tuesday that on "certain questions" both sides were narrowing their differences but that the negotiations remain "difficult and complicated".
