Bush provided no details on the nature of the new intelligence, which set off an in-depth intelligence review of the evidence and assumptions that underpinned a 2005 assessment, which had held with "high confidence" that Iran was determined to acquire nuclear weapons.

Mike McConnell, the director of national intelligence, came to Bush in August 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 said at a White House news conference in describing the encounter.

Bush and Vice President Dick Cheney were not formally briefed until Wednesday on the intelligence community's new finding that Iran had had a covert nuclear weapons program but halted it in 2003 -- a bombshell with major implications for US policy.

Democrats attacked Bush for ignoring the new intelligence on October 17 when he raised the prospect in a speech of "World War III" if Iran acquired the knowledge to make nuclear weapons.

"It's exactly what he did in the run up to the war in Iraq in consistently exaggerating intelligence suggesting that Iraq had WMD (weapons of mass destruction), while failing to tell the American people about intelligence concluding that it did not," said Senator Joe Biden, who is running for president.

The National Intelligence Estimate, as the assessment is called, judged "with high confidence that the halt lasted at least several years."

Because of "intelligence gaps," the report said, the intelligence community had "moderate confidence" that Tehran had not restarted the program as of mid-2007 and did not know whether it currently intends to develop nuclear weapons.

"Without getting into sources and methods, I believe that the intelligence community has made a great discovery, and they've analyzed the discovery, and it's now a part of our government policy," Bush said.

The Washington Post reported that the new information included intercepts of conversations between Iranian military commanders.

A senior Iranian military official was overheard complaining in one intercepted conversation that the nuclear program had been halted years earlier, the Post said, citing a source familiar with the intelligence.

It said more than 1,000 pieces of information were cited in footnotes to the 140-page classified report, which represents the consensus view of the 16 US intelligence agencies.

The new information initially was greeted with deep skepticism by top Central Intelligence Agency officials and others who feared that Iran was engaging in strategic deception to hide a continuing covert program.

"You want to make sure it's not disinformation. You want to make sure the piece of intelligence you have is real," Bush said, explaining why in October he had warned of the prospect of "World War III" over Iran's nuclear program.

"And secondly, they want to make sure they understand the intelligence they gathered. If they think it's real, then what does it mean?"

Bush went on to say that he believes Iran remains a danger because a country that once had a covert nuclear weapons program could also restart it.

Senior intelligence officials who briefed reporters Monday on condition of anonymity said the review reinforced the 2005 estimate's conclusion that Iran did have a secret weapons program, which Tehran has denied.

The new assessment concludes that international pressure and its growing isolation prompted Tehran to halt the program in 2003, showing that it was more susceptible to world opinion than previously thought.

In the same year, the United States invaded Iraq, the AQ Khan nuclear proliferation network was broken and Libya gave up its nuclear program.

But it was not known whether other world events also had had an impact on the Iranian decision in addition to UN Security Council sanctions, the intelligence officials said.

However, they said there was an "evidentiary trail" that showed that Iran's decision to halt the nuclear weapons program was in response to international pressure.

The officials also attributed the change in the assessment to stepped up intelligence gathering on Iran since 2005 and more rigorous analytical methods put into effect after the intelligence failures in Iraq.

A US intelligence estimate in 2002 that wrongly concluded that Baghdad had weapons of mass destruction helped pave the way for the US invasion of Iraq in March 2003.

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