Talks between Iran and six world powers aimed at agreeing a lasting nuclear deal have resumed in Vienna.
The two sides hope to reach an agreement by July that trims Iran's nuclear activities in exchange for an end to sanctions choking Tehran's economy.
Tuesday's negotiations are the second in monthly meetings on a comprehensive deal.
Iranian Foreign Minister Mohamad Javad Zarif is leading his country's delegation, while EU foreign policy chief Catherine Ashton negotiates at least formally on behalf of the United States, Russia, China, Britain, France and Germany.
Iranian media said Iranian Foreign Minister Mohammad Javad Zarif had cancelled a customary pre-talks dinner with Ashton on Monday evening. The official IRNA news agency quoted sources as saying it was because of Ashton's "undiplomatic" behaviour, an apparent reference to her meeting Iranian human rights activists during her first visit to Tehran 10 days ago.
Iran has long denied accusations from Western powers and Israel that it has sought to develop the capability to produce
atomic weapons under the cover of its declared civilian nuclear energy programme.
In November, Iran and the six powers struck an interim deal under which Tehran has since shelved higher-grade uranium
enrichment - a potential path to atomic bombs - and obtained modest relief from punitive economic sanctions in return.
That six-month pact was designed to buy time for hammering out a final settlement by a July deadline, under which the West wants Iran to significantly scale back its nuclear programme to deny it the capability to devise a nuclear weapon any time soon.
Zarif, who will lead Tehran's delegation, said he expects a trickier round of talks this week than the previous meeting in
mid-February as the two sides try to iron out details such as Iran's Arak heavy water reactor and levels of uranium enrichment.