By Jonathan Marcus BBC Defence correspondent

The United States is using a new kind of weapon in Afghanistan to strike at cave complexes where al-Qaeda and Taleban fighters may be hiding.

US officials have acknowledged that they have used at least two of the so-called thermobaric weapons in the mountains of eastern Afghanistan.

The weapons create a huge pressure wave which effectively sucks the air out of the lungs of anyone unfortunate enough to be within range.

But the principles behind this weapon are not new and similar weaponry was employed extensively by Russian forces during the battle for Grozny in Chechnya.

This new thermobaric bomb is one of the most recent weapons in the Pentagon's armoury.

The US used the bombs to attack targets near Gardez

Rushed through testing following the 11 September attacks, it is intended to penetrate and destroy deeply buried targets - exactly what US forces are encountering in the cave complexes of Afghanistan.

As the name implies, it works on a combination of heat and pressure applying lessons that have been widely learnt from coal mine explosions or other industrial accidents. These are often created by clouds of gas or fine particles erupting into flame.

The thermobaric weapon reproduces this situation to order, distributing a very fine cloud of explosive material throughout the target which is then ignited.

The heat and pressure effects are formidable - soldiers caught in the blast could have the air sucked from their bodies and even their internal organs catastrophically destroyed.

Thermobaric weapons are closely related to so-called fuel-air explosives - where the explosive cloud is provided by a volatile gas or liquid.

Military targets

Such weapons were widely used by Russian forces laying siege to the city of Grozny some two years ago.

Rather than air-delivered bombs, the Russian army employed 30-barrel large calibre rocket launchers firing a fuel-air explosive warhead to level the city block-by-block.

The pressure effect from the warheads killed many people sheltering in the cellars of collapsed buildings, including many civilians.

These are, by any standards horrible weapons, but US spokesmen insist that in Afghanistan they are being employed solely against military targets.