Strikes by unmanned U.S. drone aircraft on the lawless borderlands between Afghanistan and Pakistan have long been a sore point with the Islamabad government for their high civilian death toll.

The missile strike program, introduced by President George W. Bush in 2004, has been accelerated under the administration of Barack Obama. The attacks are regularly denounced by Pakistani military and political leaders.

The drone program is so delicate and controversial that U.S. government officials will not even confirm its existence.

Now, data uncovered by a British non-profit enterprise appears to back up fears that the drones are killing innocent people.

The Bureau of Investigative Journalism reports that a total of 2,292 people have been killed by U.S. missiles in the region, including as many as 775 civilians, 168 of them children.

In one bombing of a madrassah (Islamic school ) in 2006, as many as 69 children died.

The drones are meant to kill extremist leaders hiding out in the remote mountains and valleys – and, in fact, they have struck some key targets, including Baitullah Mehsud, the leader of the Pakistani Taliban, in 2009, and Ilyas Kashmiri, a top al-Qaida official, earlier this year.

Under Obama, however, the U.S. is seeking out lower-level militants, primarily in North Waziristan province.

On Wednesday, a drone strike killed as many as 21 people in the Gora Qabristan area of North Waziristan. Reportedly, a drone hits this area about once every four days.

Chris Woods of BIJ told the Daily Telegraph newspaper: "This is a military campaign run by a secret service which raised problems of accountability, transparency and you have a situation where neither the Pakistanis nor Americans are clear about any agreements in place and where the reporting is difficult."

Woods added: "All of this means that when things go wrong there is simply no redress for the families of those who have been mistakenly killed."

Some human rights activists believe drones should be illegal.

"The Obama administration must explain the legal basis for drone strikes in Pakistan to avoid the perception that it acts with impunity,” said Sam Zarifi, Asia-Pacific director of Amnesty International, according to reports.

"The Pakistan government must also ensure accountability for indiscriminate killing, in violation of international law, that occurs inside Pakistan."