His comments came ahead of a telephone conference at 1115 GMT between representatives of Britain, China, France, Germany, Russia and the United States who have put forward an incentives package in a bid to persuade Iran to freeze its uranium enrichment programme.

The Iranian response received by letter on Tuesday "doesn't really say anything other than that they are not prepared to move any further at the moment," the source said.

"The phone talks today will be to decide on a common reponse. There is the possibility of trying to send the matter back to the Security Council but there is always the problem of persuading China and Russia," he added.

"They are just hitting the ball back," another European diplomatic source said.

"We'll have to analyse whether this is just another way to gain time or if it is serious."

Iran's letter sent to EU foreign policy chief Javier Solana and circulated to the six capitals Tehran "is ready to provide a 'clear response' to your proposal at the earliest opportunity while simultaneously expecting to receive your 'clear response' to our questions and ambiguities as well.

"Undoubtedly such mutual clarification can pave the way for a speedy and transparent negotiating process with a bright prospect and provide grounds for cooperation," the letter continued.

Meanwhile US media reported that the United States and its European allies had rejected Iran's latest letter on its suspect nuclear program and plan to seek new UN Security Council sanctions.

"Our offer is clear, and their response is not clear," Ric Grenell, spokesman for the US mission to the United Nations, told the Washington Post.

"The Iranian government needs to respond in a very clear way to the generous offer (from) the international community."