The efforts of Britain, France and Germany to persuade Tehran to stop enriching uranium in exchange for technical, commercial and political advantages from Europe can only succeed "if the United States joins in and throws its weight behind it," ElBaradei told the weekly Der Spiegel.

"Progress is difficult to conceive without Washington," the head of the International Atomic Energy Agency (IAEA) said, adding, "We need a common front."

ElBaradei warned that if Iran was determined to have nuclear weapons they could be available within two to three years, as Tehran certainly had the know-how and the industrial capability.

He also voiced alarm at the prospect of a US military attack on Iran, which President George W. Bush has not ruled out. "After such an attack, the Iranians would certainly set themselves in earnest to making a bomb in secret," he said.

Bush told reporters in Washington Friday that talk of a US military strike on Iran's nuclear programs was "just not the truth" but expressed growing impatience with Tehran's response to Europe-led overtures.

Bush expressed strong support for the diplomacy by Britain, France and Germany to persuade Iran to abandon its nuclear enrichment program, but resisted calls for a bigger US role in those talks with Tehran.

Asked whether Washington would consider becoming a full, fourth partner in the talks, Bush said: "We're joined in the process" by virtue of belonging to the International Atomic Energy Agency (IAEA).

"We have made it clear that we agree with the objective to get rid of the weapons," he said. "And the United States is very pleased to be a party with you, in encouraging you to carry that message."

European officials have said that Washington has expressed growing impatience with diplomacy towards Iran and that they hope Bush will sign on more concretely to the outreach efforts led by Berlin, London and Paris.

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