WASHINGTON (Reuters) - U.S. estimates show CIA drone strikes in Pakistan's tribal areas over the last two years have killed more than 500 militants -- a fraction of whom are considered top-tier leaders -- and fewer than 30 civilians, officials said on Monday.
The number of so-called combatant and noncombatant casualties in the U.S. government tally is sharply lower than some Pakistani press accounts, which have estimated civilian deaths alone at more than 600.
Disclosing a partial tally of drone strikes since the summer of 2008, when the program was ramped up under then-President George W. Bush, could help the U.S. intelligence community counter protests from Pakistan and human rights groups about the civilian death toll.
But the numbers also show that the vast majority of the 500 killed -- more than 90 percent by some measure -- are lower-level fighters, raising questions about how much the CIA knows about each individual before they are killed, experts said.
Analysts said the truth about civilian casualties was difficult to ascertain because the region is largely inaccessible to outsiders and it is unclear what criteria the CIA uses to pick its targets and to determine who constitutes a combatant.
A U.S. counterterrorism official, who spoke on condition of anonymity, said the estimate was compiled using available intelligence as well as visual images -- presumably from the unmanned aerial drones which can circle overhead for hours after they strike to assess the damage.
There has been little independent verification of casualties in the tribal areas as reporters are generally not allowed to travel there except on occasional trips chaperoned by the Pakistani military.
Tallies by Reuters and other organizations suggest the death toll since mid-2008 was significantly higher than 500. A Reuters count showed nearly 850 people killed in at least 110 strikes since 2008.
Of those killed by the drones, roughly 14 are considered by experts to be top-tier leaders of al Qaeda, the Taliban or other militant groups, while another two dozen are deemed high-to-mid level leaders.
"Just because they're not big names doesn't mean they don't kill. They do," the U.S. counterterrorism official said. "Their facilities -- where they prepare, rest and ready weapons -- are legitimate targets, too."
The official said the CIA strikes were not "random" and are based on "information and observation, gathered over time."
Officials said U.S. civilian death toll estimates included some people who were traveling with or living with wanted militants, such as the second wife of the Pakistani Taliban's top leader. Both were killed in August by a CIA drone.
Analysts said accurately estimating civilian deaths was difficult, if not impossible, in these circumstances.
"It is unclear how you define who is a militant and who is a militant leader," Daniel Byman, a counterterrorism expert at the Brookings Institution's Saban Center for Middle East Policy, said, referring to those who associate with U.S. targets. "Do you count the driver? There are a lot of areas where people are what you might call 'half pregnant'."
(Reporting by Adam Entous; Editing by Cynthia Osterman)