"Just like September 11, only with nuclear weapons this time, that's the threat. I think that is the threat," John Bolton told ABC News's Nightline programme on Wednesday.

"I think it's just facing reality. It's not a happy reality, but it's reality and if you don't deal with it, it will become even more unpleasant."

Bolton's comments came as the five veto-holding members of the UN Security Council failed again to reach agreement on how to curb Iran's nuclear ambitions after a fifth round of negotiations.

Russia and China are resisting proposals from the US, Britain and France for a council statement that would express "serious concern" about Iran's nuclear programme and ask it to comply with demands from the International Atomic Energy Agency (IAEA).

The statement does not threaten sanctions.

At the same time, foreign ministry officials from the five powers and Germany are considering meeting in New York on Monday to review strategy, diplomats said.

Russia had previously proposed such talks in Vienna, seat of the IAEA.

Shifting focus

Wang Guangya, China's UN ambassador, said Beijing and Moscow still had problems with a proposal that the IAEA be asked to report to the Security Council within 14 days on any progress Iran makes towards meeting the UN nuclear watchdog's demands.

Workers building Iran's Bushehr
nuclear power plant

Russia and China view the reporting requirement as shifting the focus of the Iran dossier from the IAEA to the Security Council, which has the power to impose sanctions.

They would like any report on Iran's compliance to go directly to the 35-nation IAEA governing board.

"We are still discussing," Wang said after the hour-long session at the US mission to the United Nations. He said that he did not consider the talks deadlocked.

The negotiations shift to the full Security Council on Thursday when all 15 of its members are to meet for a second time to discuss the draft drawn up by France and Britain.

The draft statement also calls on Iran "to re-establish full and sustained suspension of all enrichment-related and reprocessing activities, including research and development" that the IAEA would verify.

Statement v resolution

A council statement needs to be approved by all 15 members, while a resolution requires nine votes in favour and no veto from any of the permanent members.

If the impasse continues, the West could try to force Russia and China into the uncomfortable position of having to consider a resolution.

Bolton said: "Whether it is a statement or a resolution we haven't decided.

"We're trying to hold the permanent five together first but reality is reality and time is an important factor, given that the Iranians continue to progress toward overcoming their technological difficulties in enriching uranium."