PESHAWAR, Pakistan — A stepped-up campaign of American drone strikes over the past three months has battered Al Qaeda and its Pakistani and Afghan brethren in the tribal area of North Waziristan, according to a mid-ranking militant and supporters of the government there.

The strikes have cast a pall of fear over an area that was once a free zone for Al Qaeda and the Taliban, forcing militants to abandon satellite phones and large gatherings in favor of communicating by courier and moving stealthily in small groups, they said.

The drones, operated by the C.I.A., fly overhead sometimes four at a time, emitting a beelike hum virtually 24 hours a day, observing and tracking targets, then unleashing missiles on their quarry, they said.

The strikes have sharpened tensions between the local tribesmen and the militants, who have dumped bodies with signs accusing the victims of being American spies in Miram Shah, the main town in North Waziristan, they said.