Two influential human-rights groups say they have documented dozens of civilian deaths in US drone strikes in Pakistan and Yemen, contradicting assertions by the Obama administration that such casualties are rare.
In Yemen, Human Rights Watch investigated six selected airstrikes since 2009 and concluded that at least 57 of the 82 people killed were civilians. They included a pregnant woman and three children who perished in an attack in September 2012.
In Pakistan, Amnesty International revisited nine suspected US drone strikes that occurred between May last year and July this year in North Waziristan. The group said it found strong evidence that more than 30 civilians were killed in four of the attacks.
Video: Amnesty urges US to end drone attack secrecy
The basic circumstances of each of the drone strikes had been previously reported by local and international news outlets. But the human-rights groups said they were able to shed further light on each of the incidents by interviewing survivors, other witnesses and government officials in both countries.
Most drone strikes in Pakistan and Yemen occur in geographically remote areas that are often hostile to outsiders, making independent assessments extremely difficult.
The groups' findings coincide with another report released by a United Nations human-rights investigator, who estimated that 2,200 people have been killed in drone strikes over the past decade in Pakistan alone.
Of those casualties, at least 400 were civilians and 200 "probable non-combatants", according to the UN official, Ben Emmerson. He said the statistics were provided to him by Pakistan's Foreign Ministry.
Washington almost never publicly acknowledges its role in individual drone strikes and its legal justification for targeting specific people is shrouded in secrecy.
Partly as a result, estimates of drone-related casualties vary wildly. Sorting out how many people were legitimate targets under the laws of war and how many were bystanders is an even greater challenge.
In their reports, Human Rights Watch and Amnesty International called on the Obama administration to make its drone-targeting policies more transparent and to publicly investigate reported civilian casualties.
"The full picture will only come to light when US authorities fully disclose the facts, circumstances and legal basis for each of its drone strikes," Amnesty International concluded in its report.
White House spokeswoman Caitlin Hayden declined to comment on the reports' findings. But she cited a speech given by President Barack Obama in May, when he announced newly narrowed guidelines for drone attacks. Obama said drones would only be used against people who pose a "continuing, imminent threat" to the United States and only in cases in which the avoidance of civilian casualties would be "a near-certainty".
Drone strikes in Pakistan are carried out by the CIA under a covert programme. In Yemen, the CIA and the military's Joint Special Operations Command both conduct drone attacks.
Amnesty International highlighted a July 6, 2012, drone attack in the village of Zowi Sidgi, near the city of Miram Shah, in which it said 18 civilians were killed.
In that case, a group of male labourers had gathered in a tent for dinner when a missile blast killed 10 of them. A few minutes later, as rescuers arrived to treat the wounded, another round of missiles killed another eight.