The United States, Britain, France, Germany, China and Russia are to hold talks Wednesday on the next step to take after receiving Iran's latest letter on their offer of economic and technical incentives in return for halting its nuclear drive.

But the Washington Post and New York Times said the United States had already rejected the Iranian letter.

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

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

The New York Times, quoting unnamed US and European diplomats, said that the US administration considered the Iranian letter "unacceptable" and would go back to the UN Security Council.

"As predicted, it basically says, Yes, we will give you an answer, but when you give us an answer,'" a European diplomat who spoke on condition of anonymity told the Times.

The report said the United States and its European allies planned to go back to the UN Security Council to seek new sanctions against Iran, which has refused to halt its uranium enrichment, a key stage in any effort to make a nuclear bomb.