WASHINGTON — Iran halted its nuclear weapons program four years ago, and there is no evidence it has enriched uranium to build an atomic bomb, according to declassified portions of a secret U.S. intelligence assessment released Monday — directly contradicting the Bush administration's portrayal of Iran as a terrorist state bent on developing an atomic arsenal.

The much-anticipated National Intelligence Estimate on Iran's nuclear program, ordered by Congress in 2006, concludes that Tehran's decision to halt its nuclear weapons program "suggests it is less determined to develop nuclear weapons than we have been judging since 2005." The Iranian government's decisions, according to the report, "are guided by a cost-benefit approach" rather than a rush to obtain a nuclear weapon "irrespective of the political, economic and military costs."

The report — considered the collective judgment of the nation's 16 spy agencies — estimated the earliest Iran could produce a bomb is 2010 if it resumed its weapons program, but it is more likely that the ability to make a nuclear weapon "may not be attained until after 2015."

The conclusions contrasted sharply with recent statements of President Bush, Vice President Dick Cheney and many of the top Republican presidential candidates. Most seem to regard Iran as a nation that wants a nuclear arsenal despite the cost — including a potential military confrontation with the United States.

The intelligence flip-flop recalled the embarrassing reversal that Bush was forced to make on whether Iraq possessed weapons of mass destruction. The conviction that Saddam Hussein had such weapons was one of the factors behind Bush's decision to invade Iraq. It since has been determined that Iraq did not have weapons of mass destruction.

Democrats on Monday did not hesitate to suggest an Iran-Iraq comparison.

Senate Majority Leader Harry Reid said Democrats had requested the new Iran assessment "so that the administration could not rush this Congress and the country to another war based on flawed intelligence." He said the report's conclusions lead him to believe "we should be having a surge of diplomacy with Iran" rather than a military confrontation.

The White House insisted the report proves its current strategy for dealing with Iran — diplomatic overtures combined with tough talk — is working.

National Security Adviser Stephen Hadley told reporters that the estimate "offers grounds for hope that the problem can be solved diplomatically without the use of force, as the administration has been trying to do."

Hadley insisted, however, that Iran remains a serious threat and said the international community should continue its sanctions to keep Iran from resuming its nuclear weapons program.

Donald Kerr, the Bush administration's deputy national intelligence director who oversaw the report, said it reflects that "our understanding of Iran's capabilities have changed." He said the decision to declassify portions of the report was to ensure "an accurate presentation" of the situation, and to help inform public debate on how to best deal with Iran.

Many defense analysts said the new assessment calls into question the more hawkish views of Iran and indicates that engaging Iran in the global community is more likely to control its weapons ambitions than threats of force alone.

President Bush recently raised the specter of "World War Three" should Iran be allowed to develop nuclear weapons, while Cheney recently said the international community "cannot stand by as a terror-supporting state fulfills its most aggressive ambitions."

The intelligence report "undercuts the argument for military strikes on Iran and strengthens the case for diplomatic engagement," said Joseph Cirincione, a nuclear proliferation specialist at the Center for American Progress, a left-leaning Washington think tank. "It judges with high confidence that Iran gave up dedicated nuclear weapons work four years ago and they could be convinced to give it up permanently with the right combination of incentives."

Congress requested the new estimate on Iran about 15 months ago, and it was due in spring. But as the agencies finished, intelligence officials stumbled across "more information that inserted some new questions," Michael McConnell, director of National Intelligence, said last month. The agencies took several additional months sorting through that information, he said.

Senior intelligence officials said Monday the findings were based on the new intelligence, though they declined to provide details. Some observers believe the February 2007 defection of Ali Reza Asgari — a senior Iranian general and deputy defense minister believed to have had access to sensitive information about Iran's nuclear program — may have spurred the reassessment.

Iran suspended its nuclear weapons design work and its covert efforts to produce highly enriched uranium around October 2003 — after the U.S.-led invasion of neighboring Iraq, and after an Iranian opposition group revealed a secret uranium enrichment project at a facility known as Natanz, according to the report.

Vigorous diplomatic efforts by the International Atomic Energy Agency and Germany, France, and Britain, pushed Iran to agree to suspend all uranium enrichment work, but it rejected offers of incentives in exchange for terminating its program. Since then, Iran has been accused of stonewalling international inspectors and has faced harsh sanctions as punishment. In the fall of 2005, after the election of hard-line president Mahmoud Ahmadinejad, Iran resumed enriching uranium, but the intelligence report concluded that it was solely for producing fuel for a nuclear reactor, which is allowed under the Nuclear Nonproliferation Treaty.

But Intelligence officials said the fact that Iran was willing to withstand international sanctions and condemnation to press forward with that program suggests that the Iranian regime may be unwilling to foreclose the nuclear option.

Intelligence agencies do not rule out the possibility that Iran could purchase components — highly enriched uranium or plutonium — or an entire nuclear weapon from another country, according to the estimate. Indeed, one rationale for making the findings public is to put other countries on alert. But intelligence officials downplayed the danger that Iran would make such an acquisition. "If you have a country that wants an arsenal, not just a weapon, getting bits and pieces from here and there is not going to work," said a senior intelligence official who briefed intelligence officials Monday.

The new report is likely to complicate U.S. efforts to increase the pressure on Iran through further international sanctions. Russia and China have already balked at passing another round of sanctions in the U.N. Security Council.

Jim Phillips, a Middle East analyst at the conservative Heritage Foundation, said that while the intelligence report was welcome news, he remains puzzled why Iran has ignored international incentives and refuses to fully cooperate with IAEA inspectors. "If they had truly stopped the work, why are they not cashing in and gobbling up some of the diplomatic carrots being offered to them?" he said.

But Paul Pillar, the former national intelligence officer for the Near East and South Asia, was struck by the confident tone of the assessment, especially the conclusion that Iran has halted its nuclear weapons program. "If the estimators put that right up front and they have high confidence they must have pretty good reporting to justify that," said Pillar, now a professor at Georgetown University.

Contributing: Terence Hunt, Associated Press