The meeting of P5+1 -China, Russia, the UK, the US, France and Germany-group nations was held yesterday at the EU mission here and was attended by Iran's Deputy Foreign Minister for Legal and International Affairs, Seyed Abbas Araghchi.
After the meeting, Araghchi said, "It was a fruitful meeting but we cannot claim any progress yet. It's too early to say."
The US side was attended by Under Secretary of State for Political Affairs, Wendy Sherman.
Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation and Iranian President Hassan Rouhani's brother and close aide Hossein Fereydoon along with US Energy Secretary Ernest Moniz made their maiden appearance in the talks signalling that the deal is entering a sensitive stage.
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The two officials and their negotiators spent more than five hours yesterday negotiating technical details of the nuclear talks.
The tricky negotiations have mainly been over Iranian uranium enrichment and the pace of removing sanctions, which the US wants to stagger over time.
The six powerful nations are trying to broker a deal with Iran to end a more than a decade-long standoff over the Islamic Republic's nuclear programme in return for an easing of sanctions.
Iran, however, has maintained that its nuclear programme is for peaceful purposes..
Two deadlines for a permanent agreement have been missed since a November 2013 interim deal in which Iran was given limited sanctions relief in exchange for diluting its stock of fissile material from 20 per cent enriched uranium to five per cent.