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.