Two influential human rights groups say they have freshly documented dozens of civilian deaths in U.S. 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, including a pregnant woman and three children who perished in a September 2012 attack.

In Pakistan, Amnesty International investigated nine suspected U.S. drone strikes that occurred between May 2012 and July 2013 in the territory of North Waziristan. The group said it found strong evidence that more than 30 civilians were killed in four of the attacks.

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 the incidents by interviewing survivors, other witnesses and government officials in both countries.

Most drone strikes in Pakistan and Yemen occur in remote areas that are often hostile to outsiders, making independent assessments difficult.

Explore documented drone strikes in Pakistan, Yemen and Somalia

The groups’ findings coincide with a report released Friday by a U.N. human rights investigator, who estimated that 2,200 people have been killed in drone strikes over the past decade in Pakistan.

Of those casualties, at least 400 were civilians and 200 others were “probable noncombatants,” according to the U.N. official, Ben Emmerson. He said the statistics were provided by Pakistan’s Foreign Ministry.

The U.S. government almost never publicly acknowledges its role in individual drone strikes, and its legal justifications for targeting specific people are 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 reports of civilian casualties.

“The full picture will only come to light when U.S. authorities fully disclose the facts, circumstances and legal basis for each of its drone strikes,” Amnesty International concluded in its report, titled “Will I Be Next? U.S. Drone Strikes in Pakistan.”

Caitlin Hayden, a White House spokeswoman, declined to comment on the reports. But she cited a speech by President Obama in May in which he announced narrower guidelines for drone attacks. Obama said drones would be used only 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.”

Reports published by two prominent human rights groups on Tuesday criticized the Obama administration's drone strategy, accusing the White House of indiscriminately killing civilians and ignoring international law. (Reuters)

“As the President emphasized, the use of lethal force, including from remotely piloted aircraft, commands the highest level of attention and care,” Hayden said in an e-mail.

Drone strikes in Pakistan are carried out by the CIA under a covert program. In Yemen, the CIA and the military’s Joint Special Operations Command conduct drone attacks. Spokesmen for the CIA and the Pentagon declined to comment. In each country, the number of U.S. drone strikes has dropped in the past year.

Amnesty International highlighted a July 6, 2012, drone attack in the village of Zowi Sidgi, near the city of Miran Shah, in which it said 18 civilians — including a 14-year-old boy — were killed.

In that case, a group of male laborers had gathered in a tent for dinner when a missile blast killed 10 of them. A few minutes later, as rescuers arrived at the scene to treat the wounded, another round of missiles killed eight more people, according to Amnesty.

In Yemen, Human Rights Watch singled out a Sept. 2, 2012, airstrike in the village of Sarar that blew up a minibus, killing 12 passengers, including three children and a pregnant woman. The group said the Yemeni government, which works closely with U.S. counterterrorism forces, later admitted that the attack had been a mistake and compensated families of the victims.

In most of the other drone strikes cited in the reports, the human rights groups admitted that the scenarios were much less clear-cut. They acknowledged that many of those who died were suspected to be al-Qaeda or Taliban members. In other instances, civilians died alongside armed combatants.

But in virtually all cases, the groups said, it was impossible to know whether the targets had met Obama’s threshold of posing an imminent threat to the United States, because U.S. officials have kept that information a secret.