The aging chieftains of rural villages, many of whom have wives who are much younger than them, have proved keen to accept the anti-impotency drug and in exchange give a mass of information on rebels' movements and supply routes.

The pills are the latest Central Intelligence Agency innovation to avoid giving out cash or weapons in exchange for information.

In a country where corruption is rife, the CIA claims it would be impossible to track the enemy without offering such 'bribes', which have in the past included school equipment, medicine and dental care.

Jamie Smith, a veteran of CIA covert operations in Afghanistan, told a newspaper: "You're trying to bridge a gap between people living in the 18th century and people coming in from the 21st century. So you look for those common things that motivate people everywhere."

Chieftains are offered health checks before being offered Viagra to ensure the excitement is not too much for them.