But experts say much depends on whether the claims prove true and whether Russia and China see the same threat Washington does.

Iran's announcement that it had succeeded in enriching uranium to a level used in nuclear power plants added fresh tensions to its confrontation with the West, which accuses Tehran of developing nuclear weapons.

Scott McClellan, a White House spokesman, said on Tuesday: "If the regime continues to move in the direction that it is currently, then we will be talking about the way forward with the other members of the Security Council and Germany about how to address this going forward."

The State Department said it was unable to confirm Iran's claim and some experts said even if Iran's assertions were accurate, it would still be years before the Islamic state was able to produce a nuclear weapon.

Sean McCormack, the State Department spokesman, said that Iran's statements referred to an ability to produce uranium "at a fairly low enrichment level," not the highly enriched grade that is a "critical pathway" to developing nuclear weapons.

Iranian officials said they were producing enriched uranium at the 3.5% level from a cascade of 164 centrifuges, rapidly rotating cylinders used in uranium enrichment.

UN defiance

McCormack said Iran's announcement defied the UN Security Council which last month urged Tehran to halt enrichment by the end of April and return to talks on its nuclear ambitions.

The Security Council could impose tougher resolutions and sanctions but "at this point I think that we are going to see what the days ahead bring us in terms of Iranian reaction," said McCormack.

The US has always said Iran aims
to develop nuclear weapons

He urged Iran to "choose the pathway of diplomacy as opposed to the pathway of defiance."

The spokesmen did not specify what the security council's next steps might be.

Key veto-wielding members Russia and China have resisted stronger measures.

McCormack said Iran's announcement "gives more weight to the international community to act in a concerted fashion," but several experts said it remained unclear whether Russia and China would be more willing to back sanctions.

The United States has repeatedly said Iran's nuclear programme is a cover for developing nuclear weapons, while Tehran insists it is for civilian purposes to generate electricity.

Sooner than anticipated

Although Iranian officials touted their achievement as significant, some experts were sceptical.

Robert Einhorn, a former top US non-proliferation official, told Reuters: "It's hard to know what we make of it because if in fact they really believe they've reached a milestone, which is confidence that they can operate this 164-machine cascade, then that is much sooner than we anticipated."

Rumsfeld insists that the US "is
on a diplomatic track"

Iran's announcement is "probably a premature declaration of success, perhaps done for political reasons," he said, speculating that Tehran might use the announcement to justify again suspending enrichment and resuming negotiations.

Einhorn, now with the Centre for Strategic and International Studies, said: "On the other hand, it might be designed to convey the message that at this point it will do the world no good to conduct military strikes because Iran already has this technology ... and can replicate it."

Recent news reports that the Pentagon was planning possible military strikes against Iran have increased tensions but Donald Rumsfeld, the defence secretary, insisted on Tuesday the United States "is on a diplomatic track."