The accidental killing of al-Qaida hostages in Pakistan by a U.S. drone, which President Barack Obama called a regrettable mistake, illustrates the limitations of U.S. intelligence even in a region that has been closely watched for more than a decade.

With no U.S. military troops on the ground in Pakistan, and the Pakistani government unable or unwilling to force al-Qaida out of the so-called tribal areas along the Afghan border, the U.S. relies on the CIA to act when the intelligence picture is judged to be sufficiently clear.

In this case the CIA had reason to believe a meeting of mid-level al-Qaida militants was taking place at the targeted compound, but the drone strike was not aimed at a specific individual, administration officials said. It appears to be the first known instance of a U.S. drone strike killing an American hostage.

The U.S. had the al-Qaida compound under near-continuous surveillance and picked up no indication that the American hostage, Warren Weinstein, and the Italian held with him, Giovanni Lo Porto, were present, officials said. This operation, like most others based on intelligence, involved a certain amount of guesswork.

The deaths also raise the prospect of closer congressional scrutiny of the decision-making process the CIA used in weighing the risks involved in striking a suspected al-Qaida target at this stage of the long war on terror.

Sen. Tim Kaine, D-Virginia, said the incident shows the need for more clarity about the role intelligence plays in targeting al-Qaida and for an updating of the legal authority under which Obama is using force against terrorists.

The mistaken killings also are a reminder that in May 2013 Obama said that once the U.S. combat role in Afghanistan ended in December 2014 there would be less need for drone strikes against al-Qaida targets. He also acknowledged an inherent risk of mistakes in drone strikes and said he had just signed new policy guidance requiring "near-certainty that no civilians will be killed or injured" in any U.S. attack on a terrorist target.

"The highest standard we can set," he called it.

In disclosing on Thursday the mistaken killings last January, Obama said the U.S. had devoted hundreds of hours of surveillance of the Qaida compound, leading to the conclusion that a drone strike was appropriate because the compound was deemed an enemy target; no civilians were believed present and capturing rather than killing the terrorists was judged not to be feasible. Obama did not personally approve these strikes.

Michael O'Hanlon, a defense analyst at the Brookings Institution, said the U.S. had already begun limiting its use of armed drones in Pakistan because of Pakistani concerns and exaggerated claims of civilian casualties in that country.

"We're already in that era of greater restraint," O'Hanlon said.

Rep. Devin Nunes, chairman of the House intelligence committee, said U.S. counterterrorism operations must continue, despite the risks, in light of continuing efforts by radical Islamic groups to strike U.S. interests.

"This battle is guided by our intelligence community, which has the difficult task of gathering reliable information about terrorists who inhabit dangerous parts of the world. Although our intelligence is outstanding, it is not perfect," Nunes said.

The CIA's drone strike program has killed al-Qaida leaders, Pakistani Taliban fighters and other militants hiding in its tribal regions even as it has evoked anger across Pakistan over allegations of widespread civilian casualties. Since 2004, the U.S. has carried out nearly 400 suspected drone strikes in the country, according to the New America Foundation, a non-partisan think tank which tracks the campaign. The foundation says the last U.S. drone strike in Pakistan happened on Jan. 29 and killed at least six suspected militants.

Retired Army Col. Peter Mansoor, an Iraq war veteran and a professor of military history at Ohio State University, said in an interview that civilians should realize that no military action is flawless or executed with complete information.

"You almost never can get a perfectly clean military strike where you're not going to do any damage to civilians or civilian infrastructure," he said. "The important thing by the law of war is that the gain from hitting the military target needs to be commensurate with the possibility of damage to civilians and civilian infrastructure. It's called the proportionality rule."

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Associated Press writers Nancy Benac and Julie Pace contributed to this report.