CIA drone strikes in Pakistan have killed “dozens of civilians” who had gone to help rescue victims of drone strikes or were attending funerals for the victims of previous strikes, according to a new report by British and Pakistani journalists.

Published by the London-based Bureau for Investigative Journalism, the report asserts that in the three years since President Obama took office and dramatically increased the number of strikes against high-value targets:

between 282 and 535 civilians have been credibly reported as killed including more than 60 children.  A three month investigation including eye witness reports has found evidence that at least 50 civilians were killed in follow-up strikes when they had gone to help victims. More than 20 civilians have also been attacked in deliberate strikes on funerals and mourners.

“Allegations of repeat strikes coming back after half an hour when medical personnel are on the ground are very worrying,” Christof Heyns, a South African law professor who is the United Nations Special Rapporteur on Extra-judicial Executions, told the bureau. “To target civilians would be crimes of war.”

The bureau’s findings challenge the claims made just last week by President Obama, who in a rare acknowledgement of the covert program, defended the controversial strikes.

“Drones have not caused a huge number of civilian casualties. For the most part, they have been very precise precision strikes against Al Qaeda and their affiliates,” he said during an online chat hosted by YouTube and Google+ last week. “This is a targeted, focused effort at people who are on a list of active terrorists who are trying to go in and harm Americans, hit American facilities and American bases.”

But the president’s claims that the targets are all on a list are contradicted by reports, like this one in The Wall Street Journal, that the majority of CIA strikes in Pakistan are so-called “signature strikes” targeting “groups of men believed to be militants associated with terrorist groups, but whose identities aren’t always known.”

“Targeting decisions are the product of intensive intelligence collection and observation,” a senior U.S. counterterrorism official told The New York Times in response to the bureau’s report. “One must wonder why an effort that has so carefully gone after terrorists who plot to kill civilians has been subjected to so much misinformation. Let’s be under no illusions — there are a number of elements who would like nothing more than to malign these efforts and help Al Qaeda succeed.”

The discrepancies in civilian casualty numbers underscores just how difficult it is to get credible reporting from the tribal areas, as The New York Times noted in August:

Reporters in North Waziristan, where most strikes occur, operate in a dangerous and politically charged environment. Many informants have their own agendas: militants use civilian deaths as a recruiting tool, and Pakistani officials rally public opinion against the drones as a violation of Pakistani sovereignty.

The bureau’s researchers say their report is credibly and independently sourced with the eyewitness accounts of villagers who said they saw strikes, those wounded in strikes and the family members of individuals killed in strikes.

In recent months, advocacy and journalistic organizations have lobbied the government for greater transparency about its covert drone program, specifically its legal justification for targeting U.S. citizens.

Last week the American Civil Liberties Union filed a lawsuit against the Justice and Defense Departments to release records on three U.S. citizens killed in drone attacks in Yemen last year. And in late December, The New York Times filed a lawsuit [PDF] against the Department of Justice for failing to release information under Freedom of Information Act requests records about its “legal analysis justifying the use of targeted lethal force, especially as it applies to American citizens.”