A major U.S. intelligence review has projected that Iran is about a decade away from manufacturing the key ingredient for a nuclear weapon, roughly doubling the previous estimate of five years, according to government sources with firsthand knowledge of the new analysis.

The carefully hedged assessments, which represent consensus among U.S. intelligence agencies, contrast with forceful public statements by the White House. Administration officials have asserted, but have not offered proof, that Tehran is moving determinedly toward a nuclear arsenal. The new estimate could provide more time for diplomacy with Iran over its nuclear ambitions. President Bush has said that he wants the crisis resolved diplomatically but that "all options are on the table."

The new National Intelligence Estimate includes what the intelligence community views as credible indicators that Iran’s military is conducting clandestine work. But the sources said there is no information linking those projects directly to a nuclear weapons program. What is clear is that Iran, mostly through its energy program, is acquiring and mastering technologies that could be diverted to bombmaking.

The estimate expresses uncertainty about whether Iran’s ruling clerics have made a decision to build a nuclear arsenal, three U.S. sources said. Still, a senior intelligence official familiar with the findings said that "it is the judgment of the intelligence community that, left to its own devices, Iran is determined to build nuclear weapons."

At no time in the past three years has the White House attributed its assertions about Iran to U.S. intelligence, as it did about Iraq in the run-up to the March 2003 invasion. Instead, it has pointed to years of Iranian concealment and questioned why a country with as much oil as Iran would require a large-scale nuclear energy program.

The NIE addresses those assertions and offers alternative views supporting and challenging the assumptions they are based on. Those familiar with the new judgments, which have not been previously detailed, would discuss only limited elements of the estimate and only on the condition of anonymity, because the report is classified, as is some of the evidence on which it is based.

Top policymakers are scrutinizing the review, several administration officials said, as the White House formulates the next steps of an Iran policy long riven by infighting and competing strategies. For three years, the administration has tried, with limited success, to increase pressure on Iran by focusing attention on its nuclear program. Those efforts have been driven as much by international diplomacy as by the intelligence.

The NIE, ordered by the National Intelligence Council in January, is the first major review since 2001 of what is known and what is unknown about Iran. Additional assessments produced during Bush’s first term were narrow in scope, and some were rejected by advocates of policies that were inconsistent with the intelligence judgments.

One such paper was a 2002 review that former and current officials said was commissioned by national security adviser Stephen J. Hadley, who was then deputy adviser, to assess the possibility for "regime change" in Iran. Those findings described the Islamic republic on a slow march toward democracy and cautioned against U.S. interference in that process, said the officials, who would describe the paper’s classified findings only on the condition of anonymity.

The new estimate takes a broader approach to the question of Iran’s political future. But it is unable to answer whether the country’s ruling clerics will still be in control by the time the country is capable of producing fissile material. The administration keeps "hoping the mullahs will leave before Iran gets a nuclear weapons capability," said an official familiar with policy discussions.

Intelligence estimates are designed to alert the president of national security developments and help guide policy. The new Iran findings were described as well documented and well written, covering such topics as military capabilities, expected population growth and the oil industry. The assessments of Iran’s nuclear program appear in a separate annex to the NIE known as a memorandum to holders.

"It’s a full look at what we know, what we don’t know and what assumptions we have," a U.S. source said.

Until recently, Iran was judged, according to February testimony by Vice Adm. Lowell E. Jacoby, director of the Defense Intelligence Agency, to be within five years of the capability to make a nuclear weapon. Since 1995, U.S. officials have continually estimated Iran to be "within five years" from reaching that same capability. So far, it has not.

The new estimate extends the timeline, judging that Iran will be unlikely to produce a sufficient quantity of highly enriched uranium, the key ingredient for an atomic weapon, before "early to mid-next decade," according to four sources familiar with that finding. The sources said the shift, based on a better understanding of Iran’s technical limitations, puts the timeline closer to 2015 and in line with recently revised British and Israeli figures.

The estimate is for acquisition of fissile material, but there is no firm view expressed on whether Iran would be ready by then with an implosion device, sources said.

The timeline is portrayed as a minimum designed to reflect a program moving full speed ahead without major technical obstacles. It does not take into account that Iran has suspended much of its uranium-enrichment work as part of a tenuous deal with Britain, France and Germany. Iran announced yesterday that it intends to resume some of that work if the European talks fall short of expectations.

Sources said the new timeline also reflects a fading of suspicions that Iran’s military has been running its own separate and covert enrichment effort. But there is evidence of clandestine military work on missiles and centrifuge research and development that could be linked to a nuclear program, four sources said.

Last month, U.S. officials shared some data on the missile program with U.N. nuclear inspectors, based on drawings obtained last November. The documents include design modifications for Iran’s Shahab-3 missile to make the room required for a nuclear warhead, U.S. and foreign officials said.

"If someone has a good idea for a missile program, and he has really good connections, he’ll get that program through," said Gordon Oehler, who ran the CIA’s nonproliferation center and served as deputy director of the presidential commission on weapons of mass destruction. "But that doesn’t mean there is a master plan for a nuclear weapon."

The commission found earlier this year that U.S. intelligence knows "disturbingly little" about Iran, and about North Korea.

Much of what is known about Tehran has been learned through analyzing communication intercepts, satellite imagery and the work of U.N. inspectors who have been investigating Iran for more than two years. Inspectors uncovered facilities for uranium conversion and enrichment, results of plutonium tests, and equipment bought illicitly from Pakistan — all of which raised serious concerns but could be explained by an energy program. Inspectors have found no proof that Iran possesses a nuclear warhead design or is conducting a nuclear weapons program.

The NIE comes more than two years after the intelligence community assessed, wrongly, in an October 2002 estimate that then-Iraqi President Saddam Hussein had weapons of mass destruction and was reconstituting his nuclear program. The judgments were declassified and made public by the Bush administration as it sought to build support for invading Iraq five months later.

At a congressional hearing last Thursday, Gen. Michael V. Hayden, deputy director of national intelligence, said that new rules recently were imposed for crafting NIEs and that there would be "a higher tolerance for ambiguity," even if it meant producing estimates with less definitive conclusions.

The Iran NIE, sources said, includes creative analysis and alternative theories that could explain some of the suspicious activities discovered in Iran in the past three years. Iran has said its nuclear infrastructure was built for energy production, not weapons.

Assessed as plausible, but unverifiable, is Iran’s public explanation that it built the program in secret, over 18 years, because it feared attack by the United States or Israel if the work was exposed.

In January, before the review, Vice President Cheney suggested Iranian nuclear advances were so pressing that Israel may be forced to attack facilities, as it had done 23 years earlier in Iraq.

In an April 2004 speech, John R. Bolton — then the administration’s point man on weapons of mass destruction and now Bush’s temporarily appointed U.N. ambassador — said: "If we permit Iran’s deception to go on much longer, it will be too late. Iran will have nuclear weapons."

But the level of certainty, influenced by diplomacy and intelligence, appears to have shifted.

Asked in June, after the NIE was done, whether Iran had a nuclear effort underway, Bolton’s successor, Robert G. Joseph, undersecretary of state for arms control, said: "I don’t know quite how to answer that because we don’t have perfect information or perfect understanding. But the Iranian record, plus what the Iranian leaders have said . . . lead us to conclude that we have to be highly skeptical."

Researcher Julie Tate contributed to this report.