The crisis is at a clear standoff as Iran rejects the UN Security Council resolution of December 23, which imposed limited sanctions to force it to stop enriching uranium.

The process, which uses centrifuges to make fuel for civilian nuclear power reactors but can also be used to make explosive material for atom bombs, is at the heart of US charges that Iran is hiding work to develop nuclear weapons. Tehran insists its programme is a peaceful one designed to generate electricity.

A diplomat said the best outcome, on the sidelines of the Conference on Security Policy being held in the southern German city of Munich from Friday to Sunday, would be for European Union powers Britain, Germany and France, along with Russia and China to hold a meeting with Iran.

"They might agree on some sort of framework or concept under which the Iranians will pull the plug on centrifuges for a couple of months" -- opening the door to wider talks including the United States -- and for the Security Council to take "no action for that period" on sanctions.

Moscow supports a proposal by IAEA chief Mohamed ElBaradei for a "time-out" in order to encourage further dialogue, under which Iran would suspend uranium enrichment while the United Nations held off on imposing sanctions.

But diplomacy in Munich could be stilled by the long shadow of the United States, and perhaps hardline allies like Britain, which reject such simultaneity and want any Iranian enrichment pause to come first and be unconditional in order to keep Iran from winning additional time to continue strategic fuel work.

US officials have pointed out that once Iran met its commitments to stop such fuel work, the Security Council would suspend its sanctions, as the resolution makes clear.