Reporting from Kundian, Pakistan, and Bagram, --

Their whir is unmistakable, a buzzing hum that prompts the tribespeople of Waziristan to refer to the fleet of armed U.S. drone aircraft hovering overhead as machay, or wasps.

The Khan family never heard it. They had been sleeping for an hour when a Hellfire missile pierced their mud hut on an August night in 2008. Black smoke and dust choked villagers as they dug through the rubble.

Four-year-old Zeerak's legs were severed. His sister Maria, 3, was badly scorched. Both were dead. When their cousin Irfan, 16, saw them, he gently curled them into his arms, squeezed the rumpled bodies to his chest, lightly kissed their faces, and slid into a stupor.

Drones have transformed combat against Islamic militants in Pakistan's tribal areas, the rugged belt of villages and badlands hugging the border with Afghanistan. Since 2004, analysts say, Predator and Reaper drones operated by the CIA have killed at least 15 senior Al Qaeda commanders, as well as several top Pakistani Taliban leaders and hundreds of fighters.

The small unmanned planes can hover for hours while gathering infrared camera footage. Onboard lasers pinpoint targets for supersonic Hellfire missiles or 500-pound bombs. The attacks cost no American lives.

But civilians who had nothing to do with the Taliban or Al Qaeda also die in these strikes. Calculations of how many vary widely, from fewer than 30 since 2008 to more than 700 just last year. The Pakistani government restricts access to the tribal areas and has only nominal control there. Militants seal off attack sites, and victims are buried quickly, according to Islamic tradition.

Still, the deaths inflame anti-American suspicions, particularly among middle- and upper-class Pakistanis outside the tribal areas, many of whom are convinced that Washington wants to colonize their country or wrest control of its nuclear arsenal.

Inside the poor, isolated and politically powerless tribal areas, the reaction is more nuanced. Some say bluntly that they would avenge the killing of their relatives, if they could only reach those remotely piloting the drones buzzing thousands of feet over their heads. Others say they understand the need for the program, and even support it if it helps drive out the militants. They loathe the constraints the Taliban places on everyday life.

But many don't understand why a technology that pinpoints its targets with lasers and infrared cameras can also kill innocents. And they would prefer that Pakistanis, rather than Americans, were flying the drones.

The CIA's covert Predator and Reaper drone missions over Pakistan are separate from the U.S. military's unmanned flights in Afghanistan and Iraq, U.S. military officials say.

Pakistanis are in some ways partners in the effort. U.S. conventional forces are banned from the tribal areas, but Pakistan tacitly acquiesces to Washington's reliance on drones to strike militant camps and hide-outs. The U.S. also relies heavily on the Pakistani military and intelligence services for on-the-ground information. If the intelligence is faulty, or if civilians are near militant targets, civilian casualties are almost inevitable.

"We've become sort of the counterinsurgency air force of Pakistan,'' said Micah Zenko, a national security scholar at the Council on Foreign Relations.

U.S. officials say the strikes have weakened the leadership of Al Qaeda and the Pakistani Taliban and disrupted their operations, while minimizing civilian casualties.

"We believe the number of noncombatant casualties is under 30, those being people who were near terrorist targets, while the total for militants taken off the battlefield exceeds 500," said a U.S. counter-terrorism official who spoke on condition of anonymity.

The official said the estimate was based on various types of intelligence, as well as observations made before and after the strikes, and characterized unofficial estimates as little more than guesses.

"Targets are chosen with extreme care, factoring in concepts like necessity, proportionality and an ironclad obligation to minimize loss of innocent life and property damage," the official said.

Militants have an incentive to spread stories of atrocities resulting from the drone operations but have been unable to provide any detail to back up such propaganda claims, the official added.

In August 2009, a drone strike killed Baitullah Mahsud, the Pakistani Taliban chief responsible for the assassination of former Prime Minister Benazir Bhutto. However, according to an analysis by the New America Foundation, a Washington think tank, drone attacks had missed him at least 16 times in the preceding 14 months. An estimated 280 to 410 people died in those attacks, it said. About 150 to 175 were believed to be militants. The rest were listed as "other," many of them civilians.