BRUSSELS (Reuters) - World powers have asked Iran to hold a new round of talks over its nuclear work in February, while expressing disappointment over Tehran's reluctance to schedule negotiations.

A spokesman for the EU's foreign policy chief Catherine Ashton said on Monday Iran had not agreed to her proposal, issued on behalf the United States, Russia, China, France, Britain and Germany, to meet at the end of January.

"Iran did not accept our offer to go to Istanbul on January 28 and 29 and so we have offered new dates in February," Michael Mann told a news briefing in Brussels.

"We have continued to offer dates since December. We are disappointed the Iranians have not yet agreed," he said.

Ashton is overseeing diplomatic contacts on behalf of the powers hoping to persuade Tehran to scale back its nuclear work. The six powers are concerned Iran is seeking to reach the capability to build nuclear weapons, but Tehran denies that.

Mann said, however, that Iranian negotiators have put up new conditions for resuming negotiations but that EU powers were concerned that might be a delaying tactic.

(Reporting by Justyna Pawlak and Adrian Croft; editing by Rex Merrifield)