A British media outlet says a classified Pakistani government report shows U.S. drone strikes on militant targets in Pakistan have killed many more civilians than Washington has acknowledged.
A U.S. official rejected the document's claim, saying it lacks credibility.
The non-profit Bureau of Investigative Journalism said Tuesday it obtained the Pakistani report from anonymous sources and published the full version on its website.
The document lists U.S. drone strikes between 2006 and 2009 and shows at least 147 civilian deaths from the attacks, representing about one-fifth of total fatalities. It says most of the rest were militants.
A similar study issued this month by the New America Foundation said U.S. drones killed 191 civilians in the four-year period, from a total of 1,004 fatalities. The Washington-based public policy institute said the casualty figures were based on "credible" reports mostly from Western news agencies.
In a statement provided to VOA, the U.S. official said "the notion that the United States has undertaken operations in Pakistan that resulted in the deaths of hundreds of innocent Pakistanis is ludicrous."
The official said the Pakistani document listing drone casualties is not credible because it relies "in part on erroneous media reporting."
The U.S. Central Intelligence Agency has carried out hundreds of drone strikes on militants in Pakistani tribal regions since 2004, to stop them from attacking U.S. troops in neighboring Afghanistan.
U.S. officials have said the drone strikes killed only about 50 non-combatants.
Pakistan's Federally Administered Tribal Areas have long been inaccessible to independent media, making it difficult to verify the identities of drone casualties.
Pir Zubair Shah, a former New York Times journalist who reported from Pakistan, told VOA it also is hard for Islamabad to confirm the casualties of drone attacks on militant-controlled districts.
"The government itself has as many problems of accessibility as anybody else would have, like a journalist or a human rights worker, or anyone who wants to investigate anything in the tribal areas," he said.
Shah, who is from the South Waziristan tribal region and now lives in New York, said independent access to government-controlled tribal territory is heavily restricted as well. He said Pakistani authorities block roads to prevent reporters from discovering civilian casualties caused by Pakistani military operations.
Shah said the Taliban imposes similar road restrictions to stop journalists from learning about militant training camps and its sheltering of al-Qaida terrorists from U.S. drones.
"After a typical strike, the Taliban cordons off the area. They take [away] the dead bodies and make sure that if there is an important person [among them], that he is buried as soon as [possible], especially outsiders, foreigners like al-Qaida, Uzbeks and others," he said.
Shah said the immersion of Taliban fighters into the daily life of tribal communities also has blurred the line between militants and civilians.
He said fighters often recruit teenagers and share living compounds with family members not directly engaged in combat.
“There are a lot of things in terms of definitions [and] the social structure of the tribal areas, which create confusion. These things have to be cleared [up] before we reach a conclusion on these subjects," he said.
The Pakistani government had no immediate response to the Bureau of Investigative Journalism's report.