Washington ? The Bush administration is working to codify the evolution of U.S. strategic nuclear doctrine from the Cold War policy of massive retaliation against the Soviet Union to a more flexible system that warns of a pre-emptive strike against hostile countries that threaten to use weapons of mass destruction.

The policy would give U.S. presidents the option of conducting a pre-emptive strike with precision-guided conventional bombs or nuclear weapons.

A policy being put together by the Bush administration would give U.S. presidents the option of conducting a pre-emptive strike against hostile countries that threaten to use weapons of mass destruction with precision-guided conventional bombs or nuclear weapons. The policy would be a new one for members of the U.S. military, including Sgt. Preston Ring, left, a soldier of U.S. 101st Airborne Division, who on Saturday showed Maj. Gen. Frank L. Hagenbeck the work being done on a helicopter damaged in Operation Anaconda at U.S. Air Base Kandahar in Afghanistan.

This system, which Pentagon planners call “offensive deterrence,” would put an official end to the practice of assigning a set of fixed targets for the U.S. nuclear force, the vast majority of them in the former Soviet Union. It would replace it with a more flexible targeting scheme in which weapons could be aimed at states that threaten or use chemical, nuclear or biological arms against the United States or its allies.

Reflecting the U.S. approach, British Defense Secretary Geoff Hoon on Wednesday told members of the House of Commons in London that Britain was prepared to use its nuclear weapons to respond to a biological or chemical attack.

Details of the U.S. policy are contained in excerpts from the Pentagon’s nuclear posture review, which in January was sent to Congress. The review, parts of which remain classified, is aimed at providing a blueprint for developing and using nuclear weapons and will serve as the basis for a new presidential decision directive. The last nuclear posture review was completed in 1994.

“Nuclear weapons will, of course, continue to play a vital role,” the new review says. “However, they will be just part of the (offensive strategic force), integrated with new non-nuclear strategic capabilities that strengthen the credibility of our offensive deterrence.”

The more flexible targeting system, called “adaptive planning,” was first adopted by the Clinton administration after the collapse of the Soviet Union. In the past decade, it has slowly replaced the Cold War system under which more than 1,000 U.S. nuclear warheads were assigned fixed targets in Russia.

By spending more than $1 billion during the Clinton administration on computer upgrades, the Strategic Command was able to reduce the time it takes to assign new targets for strategic weapons from months to hours, and in some cases, minutes, according to former officials.

The Bush nuclear posture review calls for making the system more flexible. “The desire to shorten the time between identifying a target and having an option available will place significant stress on the nuclear planning process as it currently exists,” the review says.

For example, to replace the major attack options against Russian targets that for years governed the role for almost all U.S. warheads on land- and submarine-based nuclear intercontinental ballistic missiles, the review talks of “immediate, potential or unexpected contingencies.”

Immediate contingencies, which require pretargeted weapons, “include an Iraqi attack on Israel or its neighbors, a North Korean attack on South Korea or a military confrontation (with China) over the status of Taiwan,” the review says.

Potential contingencies are “plausible but not immediate dangers” and include situations in which hostile states possess weapons of mass destruction and the means of delivering them. Unexpected contingencies are likened to the Cuban missile crisis or a change of government in a country that has nuclear, chemical or biological weapons.

The report mentions North Korea, Iraq, Iran, Syria and Libya as having long-standing hostility toward the United States and so could be involved in “unexpected contingencies.”

Bruce Blair, president of the Center for Defense Information, said removal of Russia from immediate weapons targeting reduces the possibility of a nuclear exchange between the two countries.

John Pike, director of GlobalSecurity.org, a defense think tank, noted that one classified paragraph in the nuclear review says that despite the changed relationship with Moscow, “Russia’s nuclear forces and programs remain a concern. U.S. planning must take this into account.”