Nuclear talks between Iran and six world powers have virtually stalled and a deadline for a final deal may have to be postponed again, Russian news agency TASS quoted a diplomatic source as saying on Friday.

Iran and the powers -- the United States, Russia, China, France, Britain and Germany -- are trying to reach a settlement by June 30 under which Iran would curb its nuclear programme in exchange for relief from economic sanctions.

The United States has said it stands by the end-June deadline for the deal, meant to assuage Western fears that Iran is working to develop a nuclear bomb.

But other officials have indicated the date might be missed as negotiations about technical details drag on.

Iran denies any ambition to develop nuclear weapons and says its programme is for generating electricity and other peaceful purposes.

The latest round of discussions in Vienna on Friday had made no significant progress, the source from one of the missions said, according to TASS.

"The process has virtually stalled, there is risk that deadline will have to be postponed again," TASS said the source added.

Among the unresolved issues are the pace of easing Western sanctions imposed over the Iranian programme and the monitoring and verification measures to ensure Iran could not pursue a clandestine nuclear weapons programme. – Reuters