A U.S. drone strike on a terrorist compound in Pakistan in January accidentally killed two Westerners being held hostage, including an American aid worker, the White House said Thursday.

The drone strike in the Pakistan-Afghanistan border region accidentally killed Dr. Warren Weinstein, an American held by al Qaeda since 2011, and Giovanni Lo Porto, an Italian national who had been held hostage since 2012. It’s believed to be the first time that the CIA’s drone operations in Pakistan have killed an American.

An American al Qaeda member also was killed in the strike, and another American-born terrorist was killed in a different operation, U.S. officials said.

At a hastily arranged appearance at the White House, President Obama said the operation was “fully consistent” with U.S. guidelines for counterterrorism strikes in the region, and he blamed the episode on the “fog of war.” He said he has ordered a review to make sure it doesn’t happen again.

“We believed that this was an al Qaeda compound, that no civilians were present, and that capturing these terrorists was not possible,” Mr. Obama said. “What we did not know, tragically, is that al Qaeda was hiding the presence of Warren and Giovanni in this same compound. It is a cruel and bitter truth that in the fog of war generally, and our fight against terrorists specifically, mistakes, sometimes deadly mistakes, can occur.”

**FILE** This image from video provided by the SITE Intelligence Group shows Adam Gadahn as he appeared on a video released on militant websites on Oct. 23, 2010. (Associated Press/SITE Intelligence Group)
**FILE** This image from video provided by the SITE Intelligence Group shows ... more >
The episode is sure to revive scrutiny by lawmakers and others about the use of drones in U.S. counterterrorism operations, which increased greatly under Mr. Obama as technology improved and the administration sought less emphasis on boots on the ground.

Mr. Obama said U.S. intelligence officials had observed the compound for hundreds of hours before launching the strike.

“I profoundly regret what happened,” Mr. Obama said. “On behalf of the United States government, I offer our deepest apologies to the families. As president and as commander-in-chief, I take full responsibility for all our counterterrorism operations, including the one that inadvertently took the lives of Warren and Giovanni.”

The White House said an analysis of all available information led intelligence officials to judge “with high confidence” that the operation accidentally killed both hostages.

The president said members of his national security team “do believe that the operation did take out dangerous members of al Qaeda.” The operation also killed Ahmed Farouq, an American who was an al Qaeda leader.

And Adam Gadahn, an American who became a prominent member of al Qaeda, was killed in January, likely in a separate U.S. government counterterrorism operation, the White House said.

“While both Farouq and Gadahn were al Qaeda members, neither was specifically targeted, and we did not have information indicating their presence at the sites of these operations,” the White House said in a statement.

Mr. Obama said he ordered that the information be declassified “because the Weinstein and Lo Porto families deserve to know the truth.” He spoke on Wednesday to Mr. Weinstein’s widow, Elaine, and to Italian Prime Minister Matteo Renzi.

A somber Mr. Obama said that although the mistake was a tragedy, “one of the things that makes us exceptional is our willingness to confront squarely our imperfections and to learn from our mistakes.”

“Already, I have directed a full review of what happened. We will identify the lessons that can be learned from this tragedy, and any changes that should be made,” he said. “We will do our utmost to ensure it is not repeated, and we will continue to do everything we can to prevent the loss of innocent lives, not just innocent Americans, but all innocent lives, in our counterterrorism operations.”