The officials spoke after Iran defied warnings by the international community and restarted work at its Isfahan plant toward enriching uranium that could be used in a nuclear bomb.

Deputy State Department spokesman Adam Ereli recalled the US vow to haul Iran before the UN Security Council if it broke the agreement reached in Paris nine months ago to suspend such activities.

But in contrast to European assessments that Iran had triggered a "grave crisis," Ereli avoided any harsh rhetoric and said Washington was looking to revive Tehran's discussions with Britain, France and Germany.

"We'll continue to work with the EU-3 in support of efforts to get this process back on track," he said at a State Department briefing.

The International Atomic Energy Agency (IAEA), the UN watchdog, was due to hold an emergency meeting Tuesday in Vienna. But diplomats said it would issue Iran a warning rather than threaten it with UN action immediately.

A senior US official, who asked not to be named, confirmed Washington was looking more to rekindle the talks with Iran than bring in the United Nations. "We're not ready to give up yet," he said.

"Let's see if we can't put this genie back in the bottle. We're going to be looking at ways with the Europeans, I think, to see if we can't walk that back a little bit from the Iranians."

Iran, which Washington accuses of seeking nuclear arms, restarted work on its nuclear fuel cycle despite the European efforts to wean Tehran off its ambitions with economic and security incentives.

The Islamic Republic rejected as "unacceptable" the latest offer Friday that would have allowed it to retain a civilian nuclear capacity but barred it from enriching uranium that could be used in a bomb.

"This is Iran thumbing its nose at a productive approach by the EU-3, and we'll have to work together to take a response," Ereli said.

The United States was initially skeptical of the European attempt and had pressed early on to bring Iran before the United Nations for possible sanctions, but lacked support.

Washington changed tack in March and backed the EU-3 with a promise to help Iran eventually obtain spare parts for civilian airliners and not to oppose its bid for entry into the World Trade Organization.

US officials have admitted the shift was part of an effort to repair ties with their European allies that were frayed by the US-led invasion of Iraq in March 2003.

But President George W. Bush's administration remained leery of the Europeans' chances of talking Iran out of its nuclear aspirations and clearly kept its distance from the latest proposal unveiled Friday.

"This is an EU-3 plan; it's not a United States plan," a senior US official said in a conference call with reporters Friday. "We don't agree with every detail of the plan, but we do agree with the fact that it's been put forward."

The official, who spoke on condition of anonymity, also stressed that the United States had not been asked to make any new commitments to Iran.

Indeed, the hands-off US attitude has raised some questions.

For example, commentators said European offers to provide fuel for Iranian reactors could run into the threat of retaliation from the United States under the trade embargo it imposed on Iran.

They said Tehran was unlikely to be impressed with security assurances from the Europeans that do not include pledges by the United States, which has troops in Iran's neighboring countries of Iraq and Afghanistan.

But Washington appeared to have made a concession to the Europeans in endorsing a proposal that would allow Iran, part of Bush's "axis of evil," to retain nuclear capacity for civilian use.

The move came while the Americans staunchly resisted a similar concession to North Korea in six-party talks aimed at persuading Pyongyang to renounce its admitted nuclear-weapons program.

Asked why the United States appeared to trust Iran to maintain a civilian nuclear potential but not North Korea, Ereli insisted the two cases were very different.

He said North Korea was far more advanced in its nuclear activities than Iran, which experts say might need five to 10 more years to develop a bomb.

"That's one difference between the two programs; there are many others," Ereli said. "North Korea is a different country than Iran. It has a different history and all that sort of stuff."