He said that there were "still wide differences" between Britain, France, Germany and the United States on the one hand, and Russia and China on the other, regarding what the sanctions should be in the face of Iran's persistent refusal to comply with UN demands that it halt uranium enrichment.

Noting that the issue was still being discussed between capitals, the British ambassador said: "I think it unlikely we will be able to make progress during 2007."

Complicating the bargaining on tougher sanctions has been a new assessment by the US intelligence community declaring with "high confidence" that Iran halted a secret nuclear weapons program in 2003 in response to world pressure.

The assessment in a National Intelligence Estimate (NIE) overturned long-held US policy assumptions that Iran is bent on obtaining nuclear weapons, regardless of international demands or sanctions.

"We will come back to this issue in 2008," Sawers told reporters, ruling out negotiations among UN ambassadors of the six major powers here this year.

But he made clear that "whatever interpretation is being put on the US NIE, we all recognize just how serious a security threat Iran's nuclear program is, continues to be because of their persistence in developing the most sensitive nuclear technology in defiance of the Security Council's unanimous demand that they should cease."

The United States and its key Western allies suspect Iran is trying to build nuclear arms under the cover of its civilian program, a charge denied by Tehran.

Western powers want to keep up the pressure on Iran to come clean about the full extent of its nuclear program.

But China and Russia have reacted more coolly to calls to impose a third set of Security Council sanctions on Iran for its refusal to halt uranium enrichment, seen as a possible pathway to a nuclear weapons capability.

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