For 10 months, Washington has backed the bid by Britain, France and Germany to use a package of economic and other incentives to wean Tehran off suspected plans to develop a nuclear bomb.
But the patience of US officials with the initiative had clearly worn thin after the Islamic republic announced Tuesday it was resuming sensitive nuclear fuel research suspended for two years.
Washington branded the move a "serious escalation" of the dispute with Iran and said it had begun an intensive round of consultations with its allies and others on the next step.
"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," said White House spokesman Scott McClellan.
US officials had privately made no secret of their skepticism over the "EU-3’s" negotiating efforts begun in late 2004 which Washington embraced in March after a European swing by President George W. Bush.
One senior official, who asked not to be named, acknowledged the support was more of a matter of shoring up trans-Atlantic unity ruptured by the war in Iraq than a realistic hope the Europeans would succeed.
But the Americans now appear convinced their strategy of letting the talks run their course has borne fruit in highlighting Tehran’s intransigence and winning support among countries previously reluctant to act.
State Department spokesman Sean McCormack said if Tehran had "a lot of diplomatic running room" a year ago, the situation had changed dramatically through "careful diplomacy" by the United States and European Union.
Along with hardline talk from Tehran on the nuclear question and Israel that alarmed many nations, "the end result of all that has been that the Iranians are now isolated on this issue," McCormack said Tuesday.
"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," he added. "I’m not going to make any pronouncements."
US officials took heart in the recent move by the other four permanent members of the UN Security Council, China Russia, Britain and France, to demand Iran maintain its suspension of uranium enrichment activities.
"You now have Iran defying a request from Security Council members. You didn’t have that before," said one senior official, speaking on condition of anonymity.
The United States and its allies seemed to be orchestrating a diplomatic ballet on Iran, with the EU-3 to meet Thursday and reports of a possible emergency meeting of the International Atomic Energy Agency (IAEA) in two weeks.
The Vienna-based UN nuclear watchdog said in September Iran had violated its international obligations. The Americans say they have a majority on the IAEA’s 35-member board to haul Tehran before the UN Security Council.
But the question remained whether countries such as China and Russia, which hold a veto on the council, would agree to sanctions on an Iranian regime that insists its nuclear program is strictly peaceful.
Beyond that, the options seemed limited for Washington, which has little bilateral leverage with Iran after cutting diplomatic and economic ties because of the 1979 seizure of US hostages in Tehran.
US officials have floated the prospect that members of the European Union and other countries could impose their own bilateral sanctions on Iran but the idea has found little traction among allies.
The White House reiterated Tuesday that while it favored diplomacy with Iran and had no plans to attack the Islamic Republic, the possibility of using force remained on the table.
But even staunch US partners have little stomach for such an operation. British Foreign Secretary Jack Straw said Tuesday that "military action is not on our agenda, and I don’t believe it’s on anyone else’s agenda."
Ray Takeyh, an analyst with the Council on Foreign Relations, said the failure of the European negotiations left Washington seeking to craft as large an IAEA consensus as possible for referral to the United Nations.
"I don’t think alternatives are being seriously contemplated," he said.