By Paul Reynolds
World Affairs Correspondent, BBC News website
Iran says its nuclear programme is for purely peaceful purposes
An intense period of diplomatic activity is underway as Western countries try to get a Security Council resolution ordering Iran to suspend its enrichment of uranium.
But they are having trouble in getting Russia and China on board.
The moves follow an assessment from the UN's nuclear agency the International Atomic Energy Agency (IAEA) that Iran has not complied with the Security Council's current demand for a suspension of enrichment activities.
Western officials argue that Iran has been given an opportunity and that further pressure should now be applied.
However that is easier said than done.
The United States, Britain, France and Germany want to turn the current demand on Iran into an order. An agreed statement from the Council president issued on 29 March would become a formal council resolution having binding effect.
However, Russia and China, both veto holders, are reluctant to see too much pressure on Iran and are refusing to sign up to the mandatory resolution as drafted.
And they will not countenance the threat of sanctions. While not happy with Iran, they do not want an escalation of tension.
The proposed new resolution would be written under what is known as Chapter VII of the UN Charter. Under this chapter, the Security Council can "determine the existence of any threat to the peace, breach of the peace, or act of aggression and shall make recommendations, or decide what measures shall be taken in accordance with Articles 41 and 42, to maintain or restore international peace and security".
If Iran ignored a Chapter VII resolution, then Article 41, which deals with economic sanctions, might be raised. It is unlikely that Article 42, military action, will ever get mentioned. Russia, China and others would strongly oppose it.
In any case, sanctions, and beyond that military action, would require further decisions by the Security Council.
British officials involved in the negotiations say it will be a long and hard effort to get Russia and China to agree even a formal demand on Iran.
The US is pushing for touch action against Iran, including sanctions
They say that as a sweetener, Iran will be offered help with establishing the civil nuclear power programme it says it wants to develop, as long as it does not do the enrichment of fuel itself.
"We want to give Iran a clear choice," said one British diplomat.
The US and its allies will propose targeted sanctions if Iran refuses to comply. These would stop Iranian officials travelling to certain countries and restricting the sale of goods which could have a military as well as a civilian application. They might also propose an embargo on arms sales.
If the UN route leads nowhere, then the US will try to organise a "coalition of the willing" to impose unilateral sanctions.
However, Iran seems quite capable of simply ignoring all this and carrying on anyway. It appears determined to master the art of enrichment, while arguing that it does not intend to use this technology to make a nuclear bomb.
It is allowed by its membership of the Treaty on the Non-Proliferation of Nuclear Weapons (NPT) to enrich but not to make a bomb, so would have to leave the treaty if it did so.
However, because it hid an enrichment programme for 18 years, confidence in Iran has been lost on the IAEA and that is why there is this demand for a sustained suspension to allow further talks to take place.
The question on everyone's mind is of course whether, in due course, the United States would attack Iran's nuclear installations. There is no answer to that at this stage. The Pentagon is reported to have made contingency plans but it makes many such contingency plans and the United States is a long way from making any decision.