WASHINGTON • As U.S. and Israeli officials talk publicly about the prospect of a military strike against Iran's nuclear program, one fact is often overlooked: U.S. intelligence agencies don't believe Iran is actively trying to build an atomic bomb.

A highly classified U.S. intelligence assessment circulated to policymakers early last year largely affirms that view, originally made in 2007. Both reports, known as national intelligence estimates, conclude that Iran halted efforts to develop and build a nuclear warhead in 2003.

The most recent report, which represents the consensus of 16 U.S. intelligence agencies, indicates that Iran is pursuing research that could put it in a position to build a weapon but that it has not sought to do so.

Although Iran continues to enrich uranium at low levels, U.S. officials say they have not seen evidence that has caused them to significantly revise that judgment. Senior U.S. officials say Israel does not dispute the basic intelligence or analysis.

But Israel appears to have a lower threshold for action than Washington. It regards Iran as a threat to its existence and says it will not allow Iran to become capable of building and delivering a nuclear weapon. Some Israeli officials have raised the prospect of a military strike to stop Iran before it's too late.

It's unclear how much access U.S. intelligence has inside Iran, a problem that bedeviled efforts to determine whether Iraq had weapons of mass destruction before the U.S.-led invasion in 2003.

The assessment that Saddam Hussein had secretly amassed stockpiles of chemical and biological weapons and was seeking to build a nuclear weapon, cited by the administration of George W. Bush to justify the invasion, turned out to be wrong.

Iran barred inspectors from the International Atomic Energy Agency, the U.N.'s nuclear watchdog group, from visiting Parchin, a military site, last week to determine whether explosives tests were aimed at developing nuclear technology.

An IAEA report in November cited 'serious concerns" about "possible military dimensions to Iran's nuclear program," but did not reach hard conclusions. Another IAEA report is imminent.

Iran's supreme leader, Ayatollah Ali Khamenei, insisted Wednesday that Iran had no intention of producing nuclear weapons. In remarks broadcast on state television, he said that "owning a nuclear weapon is a big sin."

But he added that "pressure, sanctions and assassinations" would not stop Iran from producing nuclear energy for peaceful purposes. The U.S. and European Union have imposed strict sanctions on Iran's oil and banking sectors, and unidentified assassins on motorcycles have killed several nuclear scientists in Iran, attacks that Iran has blamed on Israel.

For now, U.S. military and intelligence officials say they don't believe Iran's leadership has made the decision to build a bomb.

"I think they are keeping themselves in a position to make that decision," James R. Clapper Jr., director of National Intelligence, told the Senate Armed Services Committee on Feb. 16. "But there are certain things they have not yet done and have not done for some time."

Clapper and CIA Director David Petraeus told a separate Senate hearing that Iran was enriching uranium below 20 percent purity. Uranium is considered weapons grade when it is enriched to about 90 percent purity, although it is still potentially usable at lower enrichment levels.

U.S. spy agencies also have not seen evidence of a decision-making structure on nuclear weapons around Khamenei, said David Albright, who heads the nonprofit Institute for Science and International Security and is an expert on Iran's nuclear program.

Albright's group estimates that with the centrifuges Iran already has, it could enrich uranium to sufficient purity to make a bomb in as little as six months, should it decide to do so.

It is not known precisely what other technical hurdles Iran would have to overcome, but Albright and many other experts believe that if it decides to proceed, the country has the scientific knowledge to design and build a crude working bomb in as little as a year. It would take as long as three years, Albright estimated, for Iran to build a warhead small enough to fit on a ballistic missile.

Albright said a push by Iran to build a nuclear weapon would probably be detected.

Some developments have bolstered the view that Iran is secretly pursuing a weapon.

In 2009, Western intelligence agencies discovered a clandestine underground facility called Fordow, near the city of Qom, that is said to be capable of housing 3,000 centrifuges needed to enrich uranium.

Israel worries that such facilities may be invulnerable to conventional bombing if Iran begins building a weapon.