The assessment says Iran continues to enrich uranium and still could develop a nuclear weapon between 2010 and 2015.
But the report, a composite of findings from U.S. intelligence agencies called the National Intelligence Estimate on Iran, should bolster those who say President George W. Bush has overstated the threat posed by Iran and weaken the argument for military action.
"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," says the unclassified summary of the secret report released Monday.
A spokesman for Iran's U.N. mission declined to comment.
It was uncertain whether the development will have an impact on the Bush administration's drive for new sanctions against Iraq in the United Nations. A top U.S. diplomat said Monday that China may be open to discussing fresh U.N. Security Council sanctions against Iran. Like Russia, China had been reluctant to support new sanctions; both Russia and China have Security Council vetoes that could stop an American effort to push through tougher sanctions.
Tensions have been escalating between the United States and Iran, which Bush in said in 2002 belonged in an "axis of evil" with Kim Jong Il's North Korea and Saddam Hussein's Iraq . Speaking of Iran during a news conference last month, Bush said, "If you're interested in avoiding World War III, it seems like you ought to be interested in preventing them (Iran) from having the knowledge necessary to make a nuclear weapon."
Rand Beers, who resigned from Bush's National Security Council just before the Iraq war in 2003 and served as security adviser to Democrat John Kerry's effort to unseat Bush in the 2004 presidential campaign, said the report should derail any appetite for war on the administration's part and should reinvigorate regional diplomacy. "The new NIE throws cold water on the efforts of those urging military confrontation with Iran ," Beers said.
Senior intelligence officials said Monday they failed to detect Iran's halt in nuclear weapons development several months after the U.S.-led invasion of Iraq in 2003, in time to reflect it in the last NIE, issued in 2005.
One of the officials said Iran is the most challenging country to spy on, harder even than North Korea, a notoriously closed society. "We put a lot more collection assets against this," the official said, "but gaps remain." The officials spoke on condition of anonymity because of the sensitivity of the subject.
Some of the changes in the new report reflect the use of "open source" intelligence, public information from sources such as the news media and international organizations. An official said, for example, that photos taken at Iran's Natanz nuclear facility during U.N. inspections in 2002 were particularly useful in assessing the capabilities of the civilian uranium enrichment program.
U.S. National Security Adviser Stephen Hadley said the risk of Iran acquiring a nuclear weapon remains "a serious problem." The estimate suggests Bush "has the right strategy: intensified international pressure along with a willingness to negotiate a solution that serves Iranian interests, while ensuring the world will never have to face a nuclear-armed Iran," Hadley said. He was less interested in what the 2005 assessment missed than what it got right: that Iran had a covert nuclear program.
Bush was briefed on the 100-page document on Nov. 28. National Intelligence Estimates represent the most authoritative written judgments of all 16 U.S. spy agencies. Congress and other executive agencies were briefed Monday, and foreign governments will be briefed beginning Tuesday, the officials said.
Despite suspension of Iran's weapons program, it may be difficult to ultimately dissuade Tehran from developing a nuclear bomb because Iran believes such a weapon would give it international prestige and leverage to achieve its national security and foreign policy goals, the assessment concluded.
"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," Hadley said.
The intelligence officials said they do not know all the reasons why Iran halted its weapons program or what might trigger its resumption. They said they are confident that diplomatic and political pressure played a key role, but said the U.S. invasion of Iraq, Libya's termination of its nuclear program and the implosion of the illegal nuclear smuggling network run by Pakistani scientist A.Q. Khan might also have influenced the Islamic republic.
To develop a nuclear weapon, Iran needs to design and engineer a warhead, obtain enough fissile material, and build a missile or other delivery vehicle. The intelligence agencies now believe Iran halted warhead engineering in 2003 and as of mid-2007 had not restarted it.
Iran is still enriching uranium for its civilian nuclear reactors that produce electricity. That leaves open the possibility that fissile material could be diverted to covert nuclear sites to produce highly enriched uranium for a warhead. Engineers have known the design for a nuclear weapon for 60 years. The countdown to a nuclear weapon is determined more by the availability of fissile material than anything else, the officials said.
Even if the country went all out with present enrichment capability, it is unlikely to have enough until late 2009 or 2010 at the earliest, the officials said. The State Department's Intelligence and Research office believes the earliest likely time it would have enough highly enriched uranium would be 2013. But all agencies concede Iran may not have sufficient enriched uranium until after 2015.
Iran would not be able to technically produce and reprocess enough plutonium for a weapon before about 2015, the report says. But ultimately it has the technical and industrial capacity to build a bomb "if it decides to do so," the intelligence agencies found. They said Iran 's immediate intentions are a mystery.
Director of National Intelligence Mike McConnell decided last month that major judgments of NIEs should not be declassified and released. The intelligence officials said an exception was made in this case because the last assessment of Iran's nuclear program in 2005 has influenced public debate about U.S. policy toward Iran and must be updated to reflect the latest findings.