By Paul Reynolds
World Affairs correspondent, BBC News website
Sanctions against Iran would herald the start of a new era of confrontation - without being certain of achieving their aim of ending Iran's nuclear ambitions.
Getting agreement at the Security Council would be hard
Any sanctions would be mainly economic. Their effect is questionable. And there are several stages to be gone through before they could be imposed.
To start with, Western countries have to agree among themselves that Iran's decision to remove UN seals and resume uranium enrichment research crosses a red line and means the end, for now at least, of diplomacy.
The EU three - Britain, France and Germany - which have been negotiating with Iran have reached that conclusion.
Then the West and its allies have to persuade the wider membership of the UN nuclear supervisory agency, the International Atomic Energy Agency (IAEA), to agree to refer Iran to the UN Security Council.
Then the Council, if it is sufficiently united, is likely only to issue a warning to Iran before taking any other action. It would probably tell Iran to suspend all activities again and re-enter negotiations.
Only if Iran refused would sanctions then be drawn up.
What kind of sanctions? They would be trade-orientated, aimed primarily at the one major industry that Iran has - its oil and gas.
EU foreign ministers have called for action on Iran's nuclear activity
Iran is also currently applying for membership of the World Trade Organization (WTO), so this application would be blocked. The application is at an early stage. A working party to examine it was set up only last year but all that work would in effect stop.
The United States already embargoes major trade with Iran. It is especially keen to stop US oil companies from helping Iran develop its reserves.
Oil and gas are by far Iran's largest exports. According to the WTO, they and mining products account for 86% of Iranian exports.
But the US could not expect other countries to take such drastic action and it might be difficult to persuade some of them to take much action at all.
For example China, a veto-holding permanent member of the Security Council and in search of oil worldwide, would hardly vote for an oil embargo - given that in November 2004, it reached a major agreement with Iran to buy its oil and gas in a deal valued by the Chinese at $70bn.
The West also has to tread carefully in the current oil crisis. At the moment Japan is the largest importer of Iranian oil and would not want the trade to be curtailed too much.
The attitude of Russia, which is building a nuclear power station for Iran, is important.
Russia's attitude has become more critical of Iran recently, notably since Iran spurned its offer to enrich uranium on behalf of Iran to provide the fuel for the nuclear power Iran says its needs and wants.
But even so, Russia might be reluctant to go too far.
So there is some way to go on the sanctions front. And would sanctions work?
It has to be doubted in the short-term. Although Iranian industry might be slowed as a result, the country's political and religious leadership does not seem to worry too much. Oil is in great demand and Iran has plenty of it.
The leadership has its eyes set on wider horizons. Iran, it says, has the right to develop its own nuclear fuel cycle and it must stand up to what is seen as Western pressure.
When politics and national pride come to the fore, economics take a back seat. And within Iran this is already an issue of national pride.
Iran has broken the international seals at the Natanz plant
Iran is astutely exploiting its legal rights in this - under the Nuclear Non-proliferation Treaty (NPT) it can indeed develop a nuclear fuel cycle under inspection. That is all it says it wants to do.
The counter-argument is that Iran forfeited that right by hiding an enrichment programme before and cannot now act as if nothing had happened. It could, this argument runs, buy fuel from well-regulated suppliers, as others do.
And what if sanctions do not work?
At some stage Iran might reach the point at which it had mastered the technology of fuel enrichment. This, as experts like the former UN weapons inspector Hans Blix has said, might take many years. But sooner or later, it could happen.
Military action might then get onto the agenda.
The West, and Israel, say that Iran cannot be trusted and that it matters because the technology used to enrich uranium for fuel can also be used to enrich it further for a nuclear explosion.
If you master one, you master the other. And that would give Iran what is known as the 'break-out' capability. It could leave the NPT and go ahead and make a nuclear device.
If that moment came, it would be another decision point for the West - and for Israel. President Bush has said time and again that he will not permit Iran to build a bomb. And Israel might not want to wait that long.