The Pentagon claims that the Active Denial System, dubbed "the people zapper", is a harmless way to control rioters or get enemies to drop their weapons.

But experts in the UK and Germany questioned the system’s safety, warning that exposure to the beam for more than a few seconds could cause extensive and potentially life-threatening second degree burns.

Dr Steve Wright, of Leeds Metropolitan University, an expert on non-lethal weapons technology, said: "There is a great worry that these weapons will redefine existing standards of cruelty."

Military officials, however, said that the system could save the lives of soldiers and innocent civilians in places like Iraq and Afghanistan. They said targeted individuals would flee the beam on reflex, avoiding sustained exposure.

During the first media demonstration of the weapon, airmen fired beams from a large dish antenna mounted on a Humvee at people pretending to be rioters and acting out other scenarios that US troops might encounter.

The crew fired beams from more than 450 metres away, nearly 17 times the range of existing non-lethal weapons, such as rubber bullets.

While the quick burst of 54C heat was not painful, it was intense enough to make participants think their clothes were about to ignite.

"This is one of the key technologies for the future," said Marine Colonel Kirk Hymes, director of the non-lethal weapons programme which helped develop the new weapon.

"Non-lethal weapons are important for the escalation of force, especially in the environments our forces are operating in."

The system uses millimeter waves, which can penetrate only 1/64th of an inch of skin, just enough to cause discomfort. By comparison, common kitchen microwaves penetrate several inches of skin.

The millimeter waves cannot go through walls, but they can penetrate most clothing, officials said. They refused to comment on whether the waves can go through glass.

Two airmen and ten reporters volunteered to be shot with the beams, which easily penetrated various layers of winter clothing.

Under the tests, volunteers were exposed to short bursts of heat only, but weapons experts cautioned that trigger-happy soldiers could override a pre-set cut-off to zap cornered targets in a long blast.

Airman Blaine Pernell, 22, said he could have used the system during his four tours in Iraq, where he manned watchtowers around a base near Kirkuk. He said Iraqis constantly pulled up and faked car problems so they could scout out US forces.

"All we could do is watch them," he said. But if they had the ray gun, troops "could have dispersed them."

The weapon is not expected to go into production until at least 2010, but all branches of the military have expressed interest in it, officials said.

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