The Moscow Treaty signed with great fanfare by Presidents George W. Bush of the United States and Vladimir Putin of Russia in May 2002 calls on both sides to reduce their strategic nuclear warheads to between 1,700 and 2,200 by 2012.

But it refers to "operationally deployed" weapons, essentially offering both governments a loophole that allows them to move an unlimited number of warheads into storage and keep them indefinitely under lock and key.

While US officials have often praised this option, Wednesday's remarks by Undersecretary of Energy Linton Brooks before the Senate Subcommittee on Strategic Forces represented the first official indication the Bush administration had actually decided to exercise it.

"The 2012 nuclear stockpile will be substantially reduced from current levels," Brooks told lawmakers. "But reductions will not lower the stockpile to 1,700-2,200 total warheads."

He said the retained warheads will be needed for routine maintenance of the arsenal, for meeting "commitments to allies," and to address threats that may arise in the future.

"In particular, sufficient warheads will be retained to augment the operationally deployed force in the event that world events require a more robust deterrent posture," Brooks argued.

The current US nuclear arsenal is estimated by experts to contain between 6,500 and 7,000 weapons.

The announcement came as the administration is pushing for a dramatic expansion of its study program focusing on so-called bunker-busting nuclear bombs that would enhance the military's ability to destroy underground command and control centers and hidden arms depots believed to exist in countries like North Korea and Iran.

Under the project, scientists are looking into whether they will be able to convert, for these purposes, two existing warheads - the B61 and the B83, officials said.

The B61 is a tactical thermonuclear gravity bomb that can be delivered by strategic as well as tactical aircraft -- from B-52 and B-2 bombers to F-16 fighter jets, experts said.

The B83 is designed for precision delivery from very low altitudes, most likely by B-2 stealth bombers.

The main task facing the scientists is finding how to harden the bombs' shells so they can survive penetration through layers of rock, steel and concrete before detonating, according to the experts.

Last year, Congress allocated 7.5 million dollars for the project. This year, the administration is seeking to boost these appropriations to 27.6 million.

"There is a clear military utility to such a weapon, which is why the Defense Department asked for it to be studied," Brooks said.

He disclosed that the Energy Department was discussing with the Air Force the effectiveness of using nuclear weapons to destroy stocks of chemical and biological weapons, but acknowledged that "no decision to study this area has yet been reached."

Experts believe an atomic blast could have the advantage of instantly incinerating chemical and biological agents, an effect that is unlikely to be achieved with conventional bombing.

The administration is also requesting 336.5 million dollars for restoring US capability to manufacture so-called plutonium pits that form the core of nuclear weapons -- an increase of 13 percent over the current year.

Although no new weapons production is currently planned, Brooks said it was important to maintain a manufacturing and scientific base for such projects.

"Our goal is to be able to design, develop, and begin production of a new warhead within three-four years of a decision to enter engineering development," he said.

WAR.WIRE