Condoleezza Rice, the US secretary of state, said on Thursday that Iran was probably the No. 1 challenge for the US, posing a significant threat due to what she thought was Tehran's support for anti-Israel fighters and meddling in Iraq.
But that threat could grow exponentially, she added.
"If you can take that and multiply it by several hundred, you can imagine Iran with a nuclear weapon and the threat they would then pose to that region," Rice told a congressional hearing.
"We may face no greater challenge from a single country than from Iran, whose policies are directed at developing a Middle East that would be 180 degrees different than the Middle East we would like to see developed," she said.
Tehran, which says its nuclear programmes are for the peaceful generation of electricity, vowed no compromise in a standoff with the West.
Washington wants the UN Security Council this month to start taking action against Iran that could lead to sanctions.
Two veto-holders on the council, Russia and China, have resisted the US push for possible sanctions and American officials have said they might have to work with nations outside of the United Nations to impose sanctions on the oil-wealthy country.
Russia's foreign minister on Thursday warned against a swift Security Council decision on Iran, pointing at Iraq as an example of what can happen if the world abandons diplomacy.
Sergei Lavrov (L) met Rice on
7 March in Washington
"We aren't reminding (everyone) who was right and who was not in Iraq, although the answer is obvious," Sergey Lavrov said in an interview on Russian state television. He said the International Atomic Energy Agency should be given more time to analyse Iran's nuclear programme.
Russia says patient diplomacy can persuade Iran to slow its nuclear programme.
A report by IAEA chief Mohammed ElBaradei on Iran's nuclear programme is being submitted to the Security Council, which must decide what steps to take. Measures could range from a mild statement urging compliance to sanctions or military measures.