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.
The next round of discussions had originally been slated for January but progress has been beset by wrangling between the two sides.
Iranian officials deny they are to blame for the delays and say Western countries are responsible for waiting until after the U.S. presidential election in November, which resulted in lost opportunities.
"We have always said that we are ready to negotiate until a result is reached and we have never broken off discussions," state news agency IRNA quoted Iranian Foreign Minister Ali Akbar Salehi as saying on Monday.
The six counties, known collectively as P5+1, met Iranian negotiators in three rounds of talks last year but made no breakthrough.
Iran has refused to halt all uranium enrichment and demanded relief from international economic sanctions before it takes any steps. But it has previously suggested it may be willing to halt higher-grade enrichment - a central concern - if its needs are met and its right to enrich is formally recognised.
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.
Salehi has suggested holding the next round of talks in Cairo but that the P5+1 wanted to meet elsewhere. He also said Sweden, Kazakhstan and Switzerland have offered to host the talks.
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