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.

Supreme Leader Ayatollah Ali Khamenei has granted the Iranian nuclear team "carte blanche" to provide guarantees to the West that the country's nuclear programme is peaceful, said a senior Iranian official who asked not to be named.

"But the red line is closure of any nuclear site and stopping enrichment," the official said. "The talks are becoming more and more difficult because hardliners in Iran are watching any outcome very closely."

He was alluding to powerful conservatives in Iran's security and clerical establishments deeply suspicious of Rouhani's diplomatic opening to the West.