The Obama administration formally acknowledged for the first time Monday its use of drone strikes against terrorism suspects, lifting but not removing the shroud of secrecy that surrounds the nation’s expanding use of targeted killing operations overseas.

Saying President Obama had instructed aides to be more open about the controversial issue, White House counterterrorism adviser John O. Brennan offered the most extensive outline yet of a clandestine program that officials had for years refused to discuss — even as evidence of its lethal toll mounted in such countries as Yemen and Pakistan.

“So let me say it as simply as I can,” Brennan said in a speech at the Woodrow Wilson Center in Washington. “Yes, in full accordance with the law — and in order to prevent terrorist attacks on the United States and to save American lives — the United States government conducts targeted strikes against specific al-Qaeda terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones.”

Brennan’s speech was also noteworthy, however, for what he withheld. He did not disclose how many people have been killed, list all the locations where armed drones are being flown or mention the administration’s increasing reliance on “signature” strikes, which allow the CIA to fire missiles even when it doesn’t know the identities of those who could be killed.

The decision to acknowledge the use of drones, and that innocent civilians have been killed, comes at a time when the administration is moving to make its national security accomplishments a central issue in the presidential campaign.

Obama has been accused in recent days of seeking to exploit for political gain the killing of al-Qaeda leader Osama bin Laden in a U.S. Special Operations raid a year ago. The president responded to the criticism during a White House news conference Monday, saying, “I hardly think that you’ve seen any excessive celebration taking place here.”

His administration has faced pressure from civil liberties groups and members of Congress to provide a fuller account of the nation’s use of drone strikes. Doing so now may enable the White House to tout its successes against al-Qaeda without having to avoid mentioning what has become a key counterterrorism tool.

Critics of the drone program described Brennan’s speech as a critical step in opening a wider debate on the issue. Until now, members of Congress could refer only elliptically to drone strikes. Even Obama was cautious in mentioning the program in an online chat with voters in January.

Brennan’s speech “is an important statement,” said Jameel Jaffer, deputy legal director at the American Civil Liberties Union, which has sued the government for greater disclosure about the use of drones. “It includes the administration’s clearest explanation thus far of the program’s purported legal basis.”

Courts have consistently sided with the administration in its efforts to guard the program’s secrecy, citing its covert status and the absence of public discussion beyond seemingly inadvertent slips. Jaffer said that may change after Brennan’s speech.

There had been extensive debate within the administration over the past year on how much to disclose about the drone program, particularly decisions to target U.S. citizens without judicial review. Three Americans were killed in Yemen last year, including alleged al-Qaeda operative Anwar al-Awlaki and his teenage son.

Early in his remarks Monday, Brennan had to pause for several minutes while a protester was removed. He then resumed with an hour-long defense of a program he likened to a scalpel.

Drones’ capability to linger over targets for days enables unprecedented “surgical precision,” Brennan said, “the ability, with laser-like focus, to eliminate the cancerous tumor called an al-Qaeda terrorist while limiting damage to the tissue around it — that makes this counterterrorism tool so essential.”

He reiterated the case made by administration lawyers over the past year that the drone program is consistent with international and U.S. law. But he went further in describing the process by which the administration makes decisions on whom it will seek to kill.

Examples of legitimate targets, Brennan said, include operational leaders of al-Qaeda, potential attackers who are actively training and militants who have “unique operational skills,” such as expertise in designing bombs that might elude airport security.

Brennan did not divulge details about how names are added to CIA and military kill lists. He said some — he did not say how many — have been rejected for not meeting criteria that the administration hopes will serve as a model for future presidents as well as other nations acquiring armed drones.

“There is, of course, no such thing as a perfect weapon, and remotely piloted aircraft are no exception,” Brennan said, acknowledging that innocent civilians have been killed but describing such cases as “exceedingly rare.”

The New America Foundation, which monitors the drone campaign in Pakistan, has estimated that civilians account for between 11 percent and 17 percent of those killed. Overall, U.S. officials have said that more than 2,000 militants and civilians have been killed in Pakistan, Yemen and elsewhere since Obama took office in 2009.

Brennan cited respect for the “sovereignty” of other countries, even though a CIA drone strike in Pakistan on Sunday came just weeks after that country’s Parliament voted unanimously to demand that such operations end.

In a question-and-answer session, Brennan declined to discuss the use of signature strikes, which are based on intelligence showing suspicious behavior rather than confirmation of the location of someone on the CIA or military target list.

The CIA has used such strikes in Pakistan for several years, but in April, Obama gave the agency and the U.S. Joint Special Operations Command authority to begin using the tactic in Yemen as well.

Brennan also signaled that more disclosures are forthcoming, saying additional files from the trove of material that was recovered from bin Laden’s compound in Pakistan will be released online this week by an organization affiliated with the U.S. Military Academy at West Point.

Staff writers Scott Wilson and Karen DeYoung contributed to this story.