By a vote of 387-36, the House of Representatives passed a spending bill on Tuesday that contains $7.5 million to research the so-called "Robust Nuclear Earth Penetrator".

One Energy Department official said the device "would enhance the nation's ability to hold hard and deeply buried targets at risk".

The Senate approved the measure by voice vote soon after. It will now go to President George Bush for signature.

Bunker-busters 

US scientists are looking into the possibility of converting into bunker-busters two existing warheads - the B61 and the B83, according to Bush administration officials.

The B61, which has selectable yields ranging from 0.3 kilotonnes to 300 kilotonnes, is a tactical thermonuclear gravity bomb that can be delivered by strategic as well as tactical aircraft.

The B83, whose yields range from one to two megatonnes, is designed for precision delivery from very low altitudes, most likely by B-2 stealth bombers, military experts said.

It has a nose cone capable of withstanding a supersonic-speed collision with concrete or steel and a delayed detonation to allow the aircraft to escape the blast.

Bush had requested $15m for
nuclear research 

High-precision strikes

The main task now is to find a way of hardening these bombs' shells to allow them to survive penetration through layers of rock, steel and concrete before detonating close to their deep underground targets.

But the $7.5 million allocated for the penetrator represents a 50% reduction from the $15 million requested by the administration.

An additional $6 million has been earmarked to study low-yield nuclear weapons that some experts believe could be useful in high-precision strikes.

Experts say that a five-kilotonne nuclear explosive detonated, for example, right on a missile silo door will vaporise the door as well as the missile inside.

Underground targets

Low-yield weapons could also be effective against other types of underground facilities such as command posts and hardened ammunition dumps.

According to the Defense Intelligence Agency, at least 10,000 such bunkers currently exist in over 70 countries around the world.

More than 1400 of them are used as strategic storage sites for weapons of mass destruction, concealed launch pads for ballistic missiles as well as leadership or top-echelon command and control posts.