WASHINGTON — In a surprising break from past statements by President Bush, a new National Intelligence Estimate on Iran concludes that the Islamic Republic’s enrichment of uranium has long been unrelated to a nuclear weapons program.

The new report finds that Iran’s nuclear weapons program halted in the fall of 2003 and that its suspension suggests the mullahs are more open to international pressure than previously believed. The prior estimate from the intelligence community concluded that Iran was pursuing a nuclear weapon. As recently as September, the president warned against Iran acquiring an atomic bomb, predicting it would set off World War III.

The finding from Washington is already having ripple effects for America’s diplomatic efforts to gin up a third U.N. Security Council resolution sanctioning Iran’s declared uranium enrichment work at Natanz.

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The director general of the International Atomic Energy Agency, Mohamed ElBaradei, has been working closely with the Iranians since August on a deal that would allow the uranium enrichment at Natanz, activity that has been sanctioned twice by the United Nations, in exchange for fuller cooperation from the Iranians on the extent of their prior nuclear program. Mr. ElBaradei has recently taken to saying he has no evidence that the enrichment at Natanz is connected to Iran’s pursuit of an atomic bomb.

The latest National Intelligence Estimate bolsters Mr. ElBaradei’s view on this as well.

A footnote draws the following distinction: “For the purposes of this Estimate, by ‘nuclear weapons program’ we mean Iran’s nuclear weapon design and weaponization work and covert uranium conversion-related and uranium enrichment-related work; we do not mean Iran’s declared civil work related to uranium conversion and enrichment.” In the estimate, the work at Natanz is referred to as Iran’s declared enrichment. The assessment judges with moderate confidence that engineers in Natanz still have trouble operating the facility.

Yesterday, Russia’s deputy ambassador at the United Nations, Konstantin Dolgov, embraced Mr. ElBaradei’s view of the Iran threat. “They have to suspend not to prevent them from enrichment for good, but basically to facilitate the process of getting the answers.” His is a view in contrast to that of America, Britain, and France, which have insisted that suspension of enrichment remains the prerequisite of further negotiations.

A former American ambassador to the United Nations, John Bolton, yesterday said the new estimate “will undercut those policies of the administration. People will argue there is no evidence of an ongoing military program. The question of where the program stood when they turned it off in 2003 is not addressed. What they have released is incomplete.”

Mr. Bolton cautioned against putting too much stock in the estimate. “There is nothing in here about the status of the military program when it was suspended,” he said. “They have high confidence it was suspended in 2003 and only medium confidence that it remains halted. This is a confession that they don’t fully know what they are talking about.”

The national security adviser, Stephen Hadley, said the report vindicates the president’s diplomatic approach. “The bottom line is this: for that strategy to succeed, the international community has to turn up the pressure on Iran — with diplomatic isolation, United Nations sanctions, and with other financial pressure — and Iran has to decide it wants to negotiate a solution,” he said in a statement.

A former aide to Mr. Bolton, David Wurmser, an Iran specialist who left Vice President Cheney’s office in September, said the assessment would make it nearly impossible to get a third U.N. Security Council resolution against Iran.

“This certainly undermines any diplomatic effort we have to corner the Iranians,” he said. “The public release of these findings, which are themselves a very unnerving development, will clearly have a grave policy impact and make it extremely difficult for us to convince our allies that there is an urgency and gravity to the nature of the Iranian threat.”

A former Gulf intelligence expert for President Clinton’s national security council, Kenneth Pollack, was less dire in his view on the impact the assessment would have on American diplomacy. “It’s probably not going to be helpful, but it is unclear how hurtful it would be,” he said, adding that a recent flap between Iran’s new nuclear negotiator and the head of the European negotiating team could temper the diplomatic repercussions of the estimate.

Two former American intelligence officials and a current official told The New York Sun that the estimate had changed significantly in the last month before its release.

“I know for a fact that most of the judgments were all hedges basically as of two months ago,” a former official said.

One explanation for the change in the estimate is the recent disclosure from Iran of some of the documents related to its weapons program to the IAEA. Between September 2003 and September 2007, Iran had stonewalled inspectors and the U.N. agency on inquiries into the history of its previously undeclared nuclear facilities.

On this point, the National Intelligence Estimate says: “We assess with high confidence that until fall 2003, Iranian military entities were working under government direction to develop nuclear weapons.”