A new US National Intelligence Estimate (NIE) released on Monday formally endorsed that conclusion, which Bush had first heard about in August from US Director of National Intelligence Michael McConnell.

On August 28, Bush warned of a "nuclear holocaust" if the Islamic republic developed nuclear weapons, and on October 17 he warned that anyone interested in avoiding "World War III" should support US pressure on Tehran.

On October 21, US Vice President Dick Cheney warned Iran to suspend uranium enrichment or face "serious consequences" -- the very language in the UN resolution on Iraq that the White House says justified the March 2003 invasion.

On Tuesday, the US president defended himself from Democratic charges of exaggerating the threat by saying that he had only been briefed on the NIE on November 28 and suggesting that McConnell's earlier warning had been vague.

"In August, I think it was, Mike McConnell came in and said, 'we have some new information'. He didn't tell me what the information was. He did tell me it was going to take a while to analyze," Bush told a press conference.

But White House spokeswoman Dana Perino said one day later that McConnell had told Bush that he was looking into information that Tehran had frozen its atomic weapons quest, casting doubt on previous US charges.

McConnell "said that if the new information turns out to be true, what we thought we knew for sure is right. Iran does in fact have a covert nuclear weapons program, but it may be suspended," the spokeswoman said.

Confronted with the apparent contradiction between her comments and Bush's own words, Perino replied: "The president could have been more precise in that language. But the president was being truthful."

But some of Bush's Democratic critics, including candidates to succeed him in the November 2008 election, charged that Bush knew or should have known in August about the blockbuster finding that upended years of US rhetoric on Iran.

"Are you telling me a president who's briefed every single morning, who's fixated on Iran, is not told back in August that the tentative conclusion of 16 intelligence agencies in the United States government said they had abandoned their effort for a nuclear weapon in '03?" asked Senator Joseph Biden.

Biden noted Bush's contention that McConnell's warning had not been specific and flatly declared: "That's not believable."

"If that's true, he has the most incompetent staff in modern American history and he's one of the most incompetent presidents in modern American history," the senator told reporters on a conference call on Tuesday.

Biden also linked the latest flap to the flawed case for war with Iraq.

"For this president to knowingly disregard or once again misrepresent intelligence about the issue of war and peace, I find it outrageous. This is exactly what he did in the run-up to the war in Iraq," said Biden.

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