``It is highly classified and it's not a weapon we choose to discuss publicly,'' Pentagon spokesman Kenneth Bacon said.

Diplomatic sources in Brussels, however, identified it as a graphite bomb, an upgrade of a weapon first used in the Gulf War to take down Iraq's power grid.

John Pike, an expert on military technologies at the Federation of American Scientists, theorized that it's probably a satellite-guided bomb containing hundreds of pounds of graphite powder with a burster charge running through it. A radar altimeter could be used to detonate the bomb at a certain height over an electrical switching station, blasting the graphite into a cloud several hundred metres across that would then settle over the targeted area. The effect will be ``throwing a lighted matchon a pool of gasoline,'' Pike said.

Because graphite is a conductor, its cloud provides short-circuit paths through the switching station which cause the station to shut itself down and burn out critical components.

The power grid can be brought back if there are back-up switching stations, and spare equipment, allowing electricity to be rerouted from power plants, ``But over time,'' Pike said, ``you could unplug a power plant from the rest of the grid.''

Daniel Goure, an expert at the Center for Strategic and International Studies, said Cruise missiles were used during the Gulf War to burst clouds of carbon filaments over Iraqi power transformers and switching stations.