"If they're not going to abide by their agreement and obligations, then we would have to look to the Security Council," White House spokesman Scott McClellan said of the possible resumption of uranium enrichment by Iran.
Washington accuses oil-rich Iran of using a civilian nuclear power programme as a cover for an effort to build atomic weapons. Tehran denies the charges.
Iran handed over a letter to the International Atomic Energy Agency (IAEA) that formally notified the UN watchdog of the imminent resumption of uranium ore conversion, the precursor to enrichment in the nuclear fuel cycle.
The move, which jeopardised months of tortuous talks with Britain, France and Germany, immediately aroused expressions of grave concern from the international community.
Iran complained that Europe has dallied too long in coming up with concrete proposals for a nuclear cooperation deal, adding that it had reason to believe that the eventual offer would be "totally unacceptable".
Iran notified the IAEA of the
resumption of nuclear activities
"Iran made an agreement, the Paris Agreement. They agreed to abide by the Paris Agreement which called for Iran to suspend their uranium enrichment and reprocessing activities while the negotiations were ongoing," McClellan said.
"The Europeans, along with the United States, believe that Iran should adhere to the Paris Agreement and continue to work with the Europeans to resolve this issue," he said.
Talking to friends
"We have made clear that if Iran is going to violate its agreement and restart uranium reprocessing enrichment activities, then we would have to look to the next step, and we would be talking with our European friends about that next step," McClellan said.
His comments came hours after US President George Bush appointed John Bolton - who has taken a hard line on Iran's nuclear programmes - to be the US ambassador to the United Nations.
A delegation of US lawmakers who were visiting the IAEA headquarters in Vienna on Monday, said Iran's move raises concerns about its intentions.
"The Europeans, along with the United States, believe that Iran should adhere to the Paris Agreement and continue to work with the Europeans to resolve this issue"
White House spokesman
"There is no reason why they should not have waited just one more week for the EU proposal. This certainly raises serious concerns about Iran's intention," said Congressman Peter King of New York.
"Of course we hope the EU-3 talks succeed," said King, a member of the House of Representatives' committee of Homeland Security.
"If not, if Iran does break the suspension of uranium conversion and enrichment we believe the (IAEA) board should report Iran to the Security Council ... for the purpose of sanctions," he said.