The Canadian army’s unmanned spy drone, the Sperwer, may sound like a domestic lawnmower but the noise is enough to pin down the Taliban in Afghanistan.

"At night when things are quiet, this is a two-stroke motor so you know it’s in the area," says Major John Casey, head of an operation using the French-made drones at the coalition air base in the southern city of Kandahar.

The effect of the sound on Taliban insurgents was demonstrated during a sweep last month by Afghan security forces in the west of Kandahar province.

To stop the rebels from breaking an Afghan cordon and escaping, the Canadians flew over the area with a Sperwer as they illuminated the area with flares.

The rebels know that the drone, made by French group Sagem, transmits real-time images of the area under surveillance by day or night. On hearing the characteristic throbbing of the aircraft, the men froze so they would not be spotted. Fourteen were eventually rounded up.

"When they hear it, they keep their head down," says Casey.

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