A Bush administration strategy announced yesterday calls for the preemptive use of military and covert force before an enemy unleashes weapons of mass destruction, and underscores the United States's willingness to retaliate with nuclear weapons for chemical or biological attacks on U.S. soil or against American troops overseas.

The strategy introduces a more aggressive approach to combating weapons of mass destruction, and it comes as the nation prepares for a possible war with Iraq.

A version of the strategy that was released by the White House said the United States will "respond with overwhelming force," including "all options," to the use of biological, chemical, radiological or nuclear weapons on the nation, its troops or its allies.

However, a classified version of the strategy goes even further: It breaks with 50 years of U.S. counterproliferation efforts by authorizing preemptive strikes on states and terrorist groups that are close to acquiring weapons of mass destruction or the long-range missiles capable of delivering them. The policy aims to prevent the transfer of weapons components or to destroy them before they can be assembled.

In a top-secret appendix, the directive names Iran, Syria, North Korea and Libya among the countries that are the central focus of the new U.S. approach. Administration officials said that does not imply that President Bush intends to use military force, covert or overt, in any of those countries. He is determined, they said, to stop transfers of weapons components in or out of their borders.

The policy sets out the practical ramifications of Bush's doctrine of preemption, contained in a national security strategy released in September, which turns away from the Cold War doctrine based on deterrence and containment. The preemption doctrine favors taking on hostile states before they can strike.

It broadens a warning that was made to Iraq on the eve of the Persian Gulf War of 1991. A letter from President George H.W. Bush promised "the strongest possible response" if Iraq were to use chemical and biological weapons against U.S. and allied troops.

But the new policy is more specific, detailing the consequences of an enemy's use of weapons of mass destruction. "The United States will continue to make clear that it reserves the right to respond with overwhelming force -- including through resort to all of our options -- to the use of WMD against the United States, our forces abroad, and friends and allies," the document says.

The timing of the document's release yesterday sends an unmistakable message to Iraqi President Saddam Hussein about the potential consequences of using nonconventional weapons in a future war.

A senior administration official, briefing reporters on the new strategy, said those options include nuclear force. The official said the 1991 letter had its intended effect. "He [Hussein] didn't cross the line of using chemical or biological weapons," the official said. "The Iraqis have told us that they interpreted that letter as meaning that the United States would use nuclear weapons, and it was a powerful deterrent."

In the past, U.S. officials saw some advantage in keeping the world guessing about how the United States would respond to evidence that a country or a terrorist group was hiding weapons of mass destruction deep underground. And Bush administration officials were at pains yesterday to insist that there is nothing new in their formulation.

Under Bush, however, Pentagon officials appear to have taken a step closer to the possible, limited use of nuclear weapons by pursuing new and more usable ones. A review of nuclear policy completed by defense officials a year ago put added emphasis on developing low-yield nuclear weapons that could be used to burrow deep into the earth and destroy underground complexes, including stores of chemical and biological arms. This has raised questions about whether the administration is lowering the threshold for using nuclear weapons.

Officials deny that they are doing so. But they also argue that the strategic calculations necessary for combating terrorism and hostile nations must inherently be different from those used during the Cold War, when deterrence meant simply convincing the Soviets that the United States, if attacked, could and would wipe them out. Against today's new enemies, the administration has argued, it may be necessary to strike preemptively and with nuclear weapons that would keep fallout to a minimum.

The administration published a broader national security strategy in September, and the preparation of a separate policy on weapons of mass destruction reflects the seriousness with which the administration takes the threat of attacks from rogue states and terrorist organizations. "Every administration seems to come under criticism for not having a strategy," the official said.

The six-page strategy released by the White House yesterday was a declassified extract of a top secret directive signed by Bush in May after resolving interagency disputes dating to January. It is among the first major policy collaborations of the National Security Council and the new Homeland Security Council, whose chairman is Tom Ridge. The classified version is identified jointly as National Security Presidential Directive (NSPD) 17 and Homeland Security Presidential Directive 4.

The new strategy does not repudiate "traditional measures" of diplomacy, multinational arms control agreements and export controls. But in its classified form, and in the interagency process that drafted it, the directive is premised on a view that "traditional nonproliferation has failed, and now we're going into active interdiction," according to one participant who spoke without authority from the White House.

Active interdiction, the official said, "is physical -- it's disruption, it's destruction in any form, whether kinetic or cyber."

Explaining the new approach, one official gave the hypothetical scenario of a ship using the Philippines as a transshipment point for special weapons to Libya. "We're going to interdict or destroy or disrupt that shipment or, during the transloading process, it is going to mysteriously disappear," the official said.

The official spoke as Spanish special forces, with U.S. intelligence support, stopped a North Korean ship bound for Yemen with Scud missiles. In rare cases, previous presidents have mounted preemptive strikes against nonconventional weapons. Those episodes, including the August 1998 missile strike on an alleged Sudanese chemical weapons plant and the bombing of some targets in Iraq four months later, have generally come in retaliation for specific enemy attacks.

Bush hinted at the new approach in a Dec. 11, 2001, speech at the Citadel, speaking of active counterproliferation. By January, a draft of NSPD 17 was circulating in the State Department, the White House, the Defense Department and the intelligence agencies. State Department officials objected to some elements of the new approach but failed to carry the decision. The Homeland Security Office, represented by policy director Richard A. Falkenrath, interjected itself as jointly responsible for managing the consequences of a successful attack on the United States. Defense Secretary Donald H. Rumsfeld, according to one participant, objected in April to language that he believed commingled military and domestic lines of authority. Bush signed the draft unchanged in May.

The intention, in theory, is not fundamentally new. The Clinton administration's Presidential Decision Directive 62, "Protection Against Unconventional Threats to the Homeland and Americans Overseas," had classified language that one former official summarized as: "If you think terrorists will get access to WMD, there is an extremely low threshold that the United States should act" militarily.

Staff writer Bradley Graham contributed to this report.