The White House says two Western hostages were killed in a U.S. antiterrorist operation near the Afghan-Pakistani border in January.
President Barack Obama expressed "tremendous sorrow" that the operation ended in the deaths of the two hostages, one of whom was a U.S. citizen and the other an Italian being held by the Al-Qaeda terrorist network.
Ahmed Farouq, a U.S. citizen who had become an Al-Qaeda leader, was killed in the same operation. Another U.S. citizen turned Al-Qaeda terrorist, Adam Gadahn, was killed in a separate operation in January.
Obama said: "Our hearts go out to the families of Dr. Warren Weinstein, an American held by Al-Qaeda since 2011, and Giovanni Lo Porto, an Italian national who had been an Al-Qaeda hostage since 2012."
He said: "No words can fully express our regret over this terrible tragedy."
A White House statement said the two hostages died when U.S. intelligence targeted the compound where they were being held without knowing the hostages were there.
Media reports cited unnamed U.S. officials as saying they were killed in a drone strike.
Weinstein, 73, was a business development expert working in Pakistan on a contract with the U.S. Agency for International Development.
He was snatched from his home in Lahore on August 13, 2011, shortly before he was due to return home after seven years working in Pakistan.
Weinstein later appeared in a video in which, under apparent coercion, he asked the United States to free Al-Qaeda prisoners.
Weinstein's widow said in a statement that "we are devastated by this news and the knowledge that my husband will never safely return home."
Italian aid worker Lo Porto, 39, disappeared in January 2012 in Pakistan.
Obama said he took "full responsibility" for the deaths and said he wanted to "provide the American people with as much information as possible about our counterterrorism operations, particularly when they take the lives of fellow citizens."
The statement did not identify which U.S. agency carried out the operation, which suggests it was carried out by an intelligence service rather than a military unit.