A new White House national security strategy document said it was the strong U.S. preference to use international diplomacy to address weapons proliferation concerns.

"If necessary, however, under long-standing principles of self-defense, we do not rule out the use of force before attacks occur, even if uncertainty remains as to the time and place of the enemy's attack," the document said.

Bush outlined the preemptive force doctrine in 2002 and many critics believe he used it as a framework for the U.S.-led invasion of Iraq three years ago over weapons of mass destruction that were never found.

Massachusetts Democratic Sen. Edward Kennedy said it was ominous that the Bush administration "would even attempt to resuscitate its radical doctrine of preventive war."

"By any standard, it's been an utter failure in Iraq and has weakened our security, not strengthened it," Kennedy said.

The United States and its European allies are locked in a test of wills with Iran over suspicions that Tehran is trying to develop a nuclear weapons program despite its insistence that it merely wants atomic power for civilian use.

"We may face no greater challenge from a single country than from Iran," said the new strategy document.

White House national security adviser Stephen Hadley defended the policy at the U.S. Institute of Peace, saying it was essential in the wake of the September 11 attacks and with militant groups seeking to acquire dangerous weapons.

"The president has made clear we need to use all the tools of national power to try to deal with and avoid that threat, but at the end of the day, effective use of military force in a smart, measured way has to remain part of your inventory," he said.

The document emphasized the need for diplomacy, while adding without elaboration: "This diplomatic effort must succeed if confrontation is to be avoided."

Bush has never taken the military option off the table, although experts believe U.S. involvement in the Iraq war is a limiting factor.

A national security expert at the Cato Institute think tank, Ted Galen Carpenter, saw a link between Bush's retention of the preemptive force policy and the talk of Iran as a major challenge.

"Highlighting Iran as the principal threat that the United States faces certainly brings the preemption doctrine back into play," he said.

Ivo Daalder, a national security expert at the Brookings Institution think tank, said he believed Bush was using the document to shift strategy away from emphasizing force to focusing more on diplomacy because he has learned that threats cannot be defeated by military force alone.

"It (the Bush administration) has been forced to change course by necessity rather than out of conviction," he said.

The document cited other concerns about Iran: that it sponsors terrorism; threatens Israel, seeks to thwart Middle East peace; disrupts democracy in Iraq, and denies freedom to Iranians. It said these can only be resolved if Iran makes the strategic decision to change its policies, open its political system and allow freedom.

"This is the ultimate goal of U.S. policy," the document said. "In the interim, we will continue to take all necessary measures to protect our national and economic security against the adverse effects of their bad conduct."

The document sought to draw a line between Iran's leaders and the Iranian people, saying "our strategy is to block the threats posed by the regime while expanding our engagement and outreach to the people the regime is oppressing."