State Department spokesman Sean McCormack accused the Iranians of doing a "bob and weave" in negotiations to persuade them to halt uranium-enrichment activities that could lead to a nuclear bomb.

"Our view is that if Iran takes an further enrichment-related steps, the international community will have to consider additional measures to constrain Iran's nuclear ambitions," McCormack told reporters.

He made his remarks after Iran announced it would resume nuclear fuel research after a suspension of more than two years, prompting the UN atomic watchdog to warn Tehran to maintain a freeze on such sensitive work.

US officials had previously drawn a red line at Iran producing enriched uranium that could be used in a bomb. But McCormack said Tehran's general evasiveness on the issue made it necessary to ban research as well.

"Trying to draw a line around something being pure research with respect to enrichment activities is not something that we're going to buy, and I don't think the international community will either," he said.

The United States has been hoping to refer Iran to the UN Security Council for possible sanctions. McCormack would not say what new measures the Americans might seek but suggested the showdown was coming to a head.

"Frankly, the patience of the international community is not infinite on this issue because it's a serious issue," he said.

Washington has been backing stalled efforts by Britain, France and Germany to wean Iran off its suspected nuclear weapons ambitions with a package of economic and security incentives.

McCormack also pointed to an offer by Moscow to enrich Iranian uranium on Russian soil as a control measure, saying it was "a very interesting proposal. We think that it is a good-faith proposal."

But both the State Department and the White House urged Tehran's Islamic regime to help build confidence in a nuclear program that Iran insists is strictly for peaceful purposes.

"They (Iran's government) made some agreements, they need to abide by those agreements and act in a good-faith way in the negotiations," White House spokesman Scott McClellan told reporters.

"They need to come clean and cooperate fully with the International Atomic Energy Agency," he said.

The IAEA urged Iran on Tuesday to "maintain its suspension of all enrichment-related activity" and said it was seeking clarification of Tehran's intentions.

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