The action has had a serious effect on the general population.

The citizens of Belgrade endured hours without electricity on Sunday night - state radio and television went off air, lifts were put out of action and hospitals were forced to use emergency generators.

The BBC's correspondent in Belgrade, Mike Williams said the effects were dramatic as lights flared for an instant and then went out for seven hours.

When power was restored there were warnings not to use electric ovens or water heaters for fear of destabilising the fragile supply system.

But Nato has said its target is the military. Without electricity, Nato spokesman Jamie Shea said, there can be "no runway lights, no computers and no secure communications".

"If a command and control system has no electricity to turn it on, it is just wire, metal and plastic, not a functioning military system," he said.

New weapon

Nato military spokesman Major General Walter Jertz of the German Air Force said on Monday that a new type of bomb, previously unused in the air campaign had been used to cause widespread disruption with minimum destruction.

Although he was unable to give more details, the weapon is thought to be a so-called "soft bomb". Unlike conventional explosives, these explode in the air above the power plants, showering splinters of graphite onto the electrical transformers below.

Graphite is a conductive form of carbon, familiar to most people as the main ingredient in ordinary pencils.

As the shards of material strike switches and power lines they cause short circuits, blowing safety devices and burning out equipment.

Similar weapons are thought to have been used by the Allies during the Gulf War to knock out power distribution to the Iraqi capital, Baghdad.

Limited damage

On this occasion, damage seems to have been concentrated on the small switching stations which control power distribution to towns and cities.

One engineer was quoted on a Yugoslav Website as saying "Nato dropped graphite bombs, not on the power plant but on the switching station".

"There's no real material damage, just short-circuiting," he said.

Nato has signalled it wants to demonstrate its ability to turn off Serbian lights whenever it chooses. Jamie Shea said on Monday that Nato had the capacity to "shut down the power system as and when we have to".

Michael Williams reports from a darkened Belgrade

That message may already be hitting home. A spokesman for the Serbian Power Company is reported to have estimated that 60% to 70% of the country was affected by the overnight power cuts.

The tactic could also make sense for Nato planners wary of the cost of rebuilding Yugoslavia's infrastructure after the conflict.