US officials also signaled mounting frustration with stalled European efforts to persuade Tehran to renounce its suspected nuclear arms ambitions and said the matter would likely end up before the UN Security Council.

State Department spokesman Sean McCormack joined other Western officials in condemning Iran's decision to end a more than two-year-old suspension on uranium research that could help develop a nuclear bomb.

"We view this as a serious escalation on the part of Iran on the nuclear issue," he said. "What you see here is the international community coming out and sending a very clear message to Iran that their behavior is unacceptable."

McCormack would not comment on the next steps but said convening an emergency board of governors meeting of the International Atomic Energy Agency (IAEA), the UN nuclear watchdog, was "certainly an option."

A senior State Department official, speaking on condition of anonymity, said: "I think we are entering a period of intense diplomatic activity on this question.

"There are intense discussions at the political director level and I would expect that you will see more and more discussions at the minister level," the official told reporters.

Officials said US Secretary of State Condoleezza Rice spoke Tuesday with Russian Foreign Minister Sergei Lavrov but would not give any details.

Iran had agreed to suspend uranium enrichment pending negotiations with Britain, France and Germany on economic and other incentives to renounce any nuclear weapons ambitions.

The White House reacted strongly to the IAEA report Tuesday that Iran had lifted the seals on its Natanz research facility and was planning to do so at two other connected sites.

"Any resumption of enrichment and reprocessing activities would be a further violation of Iran's agreements with the Europeans," spokesman Scott McClellan said.

"If the regime in Iran continues on the current course ... there is no other choice but to refer the matter to the (UN) Security Council," McClellan told reporters.

Gregory Schulte, the US ambassador to the IAEA, said earlier that Iran was "taking another deliberate step towards uranium enrichment, the process for creating nuclear bomb material.

"By cutting the seals, the Iranian leadership shows its disdain for international concern and its rejection of international diplomacy," Schulte said in a statement.

The outrage expressed by the United States and its allies came a day after the five permanent members of the Security Council urged Iran to avert a crisis and resume negotiations.

British Foreign Secretary Jack Straw called for a meeting with his French and German counterparts on Thursday and said referral of Iran to the UN Security Council would top the agenda.

The United States has long pushed for UN action against Iran, but last March came out in support of the EU-3 efforts to negotiate a solution to the nuclear standoff.

Russia has offered to conduct Iran's enrichment work on its own soil as a confidence-building measure, but Tehran has yet to take up the offer.

Washington has stepped up its rhetoric against Iran in recent months and claims it has the votes within the IAEA to haul Tehran before the UN Security Council whenever it wants to.

While the Americans had initially drawn a line at Iranian production of enriched uranium, they have said that a history of deceit and dissembling by Tehran made research activities unacceptable as well.

The US administration publicly maintained its support for the EU-3 negotiations, but officials gave signs the process could be reaching its limits. One said the backing for the European negotiators was valid "at the moment."

"As for whether the EU-3 process is the right means at this point, I think that that is a question that the EU-3 is looking at right now," McCormack said. "I'm not going to make any pronouncements."

But he added, "We think that ultimately this will end up in the Security Council just because of Iran's past behavior on this."

Still, it was an open question whether Russia and China, two permanent members of the UN Security Council with veto power, would support punitive action against Tehran.

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