Mr. Obama himself, in little-noticed remarks, has acknowledged that the legal governance of drone strikes is still a work in progress.

“One of the things we’ve got to do is put a legal architecture in place, and we need Congressional help in order to do that, to make sure that not only am I reined in but any president’s reined in terms of some of the decisions that we’re making,” Mr. Obama told Jon Stewart in an appearance on “The Daily Show” on Oct. 18.

In an interview with Mark Bowden for a new book on the killing of Osama bin Laden, “The Finish,” Mr. Obama said that “creating a legal structure, processes, with oversight checks on how we use unmanned weapons, is going to be a challenge for me and my successors for some time to come.”

The president expressed wariness of the powerful temptation drones pose to policy makers. “There’s a remoteness to it that makes it tempting to think that somehow we can, without any mess on our hands, solve vexing security problems,” he said.

Despite public remarks by Mr. Obama and his aides on the legal basis for targeted killing, the program remains officially classified. In court, fighting lawsuits filed by the American Civil Liberties Union and The New York Times seeking secret legal opinions on targeted killings, the government has refused even to acknowledge the existence of the drone program in Pakistan.

But by many accounts, there has been a significant shift in the nature of the targets. In the early years, most strikes were aimed at ranking leaders of Al Qaeda thought to be plotting to attack the United States. That is the purpose Mr. Obama has emphasized, saying in a CNN interview in September that drones were used to prevent “an operational plot against the United States” and counter “terrorist networks that target the United States.”

But for at least two years in Pakistan, partly because of the C.I.A.’s success in decimating Al Qaeda’s top ranks, most strikes have been directed at militants whose main battle is with the Pakistani authorities or who fight with the Taliban against American troops in Afghanistan.