Members of the oversight committees are limited in their ability to challenge the CIA's conclusions, a senior staff member cautioned. "I can watch video all day long -- I'm not an imagery analyst," he said. "I can only look to see if the description reasonably concurs with what my untrained eyes are seeing."

Critics argue that drone strikes sometimes hit the wrong people.

"I know for a fact that civilians are being killed in these strikes," said Brigadier Abdullah Dogar, defense attache at Pakistan's embassy in Washington. He cited a now infamous attack on March 17, 2011, in Datta Khel, North Waziristan, which the Pakistanis insist struck a tribal council and killed as many as 45 people, most of whom were not connected to Al Qaeda. The U.S. has been equally insistent that those killed were militants.

The Washington-based New America Foundation says that as many as 471 civilians have been killed in the strikes since 2004. Peter Bergen, an analyst who supervises the foundation's tally, called it "the least unreliable thing out there in the absence of any real U.S. government transparency."

But lawmakers on the House and Senate oversight committees say media reports out of the tribal areas of Pakistan and Yemen list casualties that are refuted by video evidence. For example, the BBC and other news organizations quoted local officials saying that 15 "suspected militants" were killed in the June 4 Pakistan strike that killed al Libi. But the video shows that he alone was killed, congressional aides say.

Rep. Adam B. Schiff, a Democrat from Burbank and an intelligence committee member who has watched videos of the strikes, said, "If the American people were sitting in the room, they would feel comfortable that it was being done in a responsible way."

Congressional criticism of drone strikes has been rare. But this month, 26 lawmakers, all but two of them Democrats, signed a letter to Obama questioning so-called signature strikes, in which the U.S. attacks armed men who fit a pattern of behavior that suggests they are involved in terrorist activities.

Signature strikes have been curbed in Pakistan, where they once were common, but this year Obama gave the CIA permission to conduct them in Yemen, where an Al Qaeda affiliate that has targeted the United States has established a safe haven in the south.

The lawmakers expressed concern that signature strikes could kill civilians. They added: "Our drone campaigns already have virtually no transparency, accountability or oversight."

Rep. Hank Johnson (D-Ga.) signed the letter even though he is a member of the House Armed Services Committee, which oversees some drone attacks. "We need to know that innocent people are not being haphazardly killed because of bad information," he said.

Schiff, a former federal prosecutor, said he had seen firsthand that the CIA took "extraordinary care" in its targeting.

But he also said he was not convinced, in signature strikes against military-aged males, that every person killed was a militant, suggesting some could have been tribesmen paid to help terrorists.