Washington’s reaction came after Iran removed U.N. seals at its Natanz uranium enrichment plant and resumed nuclear fuel research.
"If the regime in Iran continues on the current course and fails to abide by its international obligations there is no other choice but to refer the matter to the Security Council," White House spokesman Scott McClellan said.
If Iran started nuclear enrichment and reprocessing it would be considered a "serious escalation," he said.
But U.S. officials said a referral to the U.N. Security Council was not definite and it was unclear whether Russia and China would support such a move.
The United States may be waiting for Europe, which has taken the lead in negotiating with Iran, to announce next steps.
The United States has for more than a year failed to persuade International Atomic Energy Agency members to refer Iran to the Security Council. That has infuriated some members of Congress and pro-Israel supporters who believe bringing Iran’s case to the United Nations is long overdue.
Iran is the world’s fourth-largest exporter of crude oil and many countries are loathe to jeopardize access to these supplies.
But Tehran’s nuclear activities have long been a sore point in relations with the West, with friction increasing in recent months after President Mahmoud Ahmadinejad said the Holocaust was a myth and that Israel should be "wiped off the map."
President George W. Bush early in his administration said Iran was part of an "axis of evil," and more recently called Ahmadinejad an "odd guy."
‘IRAN IS NOT IRAQ’
The United States has repeatedly made clear it would try other diplomatic tools at the Security Council — like a council president statement — before turning to sanctions.
But some countries fear Washington will not only employ sanctions but possibly also military action, as with Iraq.
Bush has made it clear that "Iran is not Iraq," McClellan said. "We are working with the international community to resolve this in a peaceful and diplomatic manner."
The United States and its European allies have accused Iran of seeking to manufacture weapons. Tehran insists it only wants to produce civilian energy aimed at satisfying the country’s booming demand for electricity.
"Any resumption of enrichment and reprocessing activities would be a further violation of Iran’s agreement with the Europeans," McClellan said. "So such steps would be a serious escalation of the nuclear issue by Tehran."
European diplomats have said they would seek an emergency meeting of the IAEA board of governors, perhaps as early as next week. U.S. and European officials say it is most likely that Russia — and probably China too — would abstain if the IAEA board votes for referral to the Security Council.
The United States was in close consultation with France, Britain and Germany, which have been negotiating with Iran on the nuclear issue — as well as other members of the IAEA board of governors, on possible next steps, U.S. officials said.
McClellan said if Iran mastered the technology of uranium enrichment, it could apply that technology to weapons.
"The concern within the international community is well-founded," he said. "The regime in Iran has shown that it cannot be trusted."
An Iranian exile told reporters on Tuesday he had new information suggesting Tehran was farther along in being able to produce a significant bomb-grade atomic fuel at Natanz than many suspected.
Alireza Jafarzadeh, former spokesman of the National Council of Resistance of Iran, which disclosed Iran’s covert nuclear program in 2002, provided names of companies and individuals involved in the work but no real proof.