President Barack Obama apologized and said he took "full responsibility" for all counterterrorism operations, including this one.
The deaths were a setback for a longrunning US drone strike program that has targeted Islamist militants in Pakistan, Afghanistan and elsewhere, often prompting criticism inside those countries as well as from civil liberties groups in the United States.
Killed in the January operation were aid workers Warren Weinstein, an American held by al Qaeda since 2011, and Giovanni Lo Porto, an Italian who went missing in Pakistan in 2012, as well as Ahmed Farouq, an American who was an al Qaeda leader, the White House said.
Another American al Qaeda member, Adam Gadahn, was also killed, likely in a separate operation, the White House added.
"I profoundly regret what happened. On behalf of the United States government, I offer our deepest apologies to the families," Obama told reporters at the White House.
Obama said he had ordered a full review of the incident to ensure such mistakes are not repeated. Republican House of Representatives Speaker John Boehner called that "entirely appropriate" and said two House committees would likely hold hearings.
US officials said the drone strike occurred inside Pakistan in the conflict-torn border region near Afghanistan.
Use of unmanned aircraft, which enable the United States to carry out counterterrorism operations without putting US personnel directly in harm's way, has drawn criticism because of the deaths of civilians and because on occasion they have involved killing Americans abroad without judicial process.
The American Civil Liberties Union said the government should better follow its own standards before launching drone strikes. "In each of the operations acknowledged today, the US quite literally didn't know who it was killing," said Jameel Jaffer, the ACLU's deputy legal director.
'TRAGIC ERROR'
Obama said that based on the intelligence available at the time, US officials believed no civilians were present at the targeted al Qaeda compound and that capturing the al Qaeda figures was not possible. He said US officials believe the operation did kill "dangerous members of al Qaeda."
Italian foreign minister Paolo Gentiloni called the incident a "tragic and fatal error ... that our American partners made," but said Lo Porto's death was "entirely the responsibility of the terrorists."
Lo Porto's mother told reporters in Palermo: "I don't want to talk, leave me alone in my grief."
Although the operation took place in January, a US official said authorities concluded only a few days ago that the two hostages were killed.
The White House said neither Farouq nor Gadahn were "specifically targeted, and we did not have information indicating their presence at the sites of these operations."
A spokesman for al Qaeda has said Farouq was the deputy head of al Qaeda in the Indian Subcontinent. The group tried unsuccessfully last year to hijack a Pakistan warship.
Gadahn was born in Oregon, grew up in California, converted to Islam at 17 and became a spokesman and translator for al Qaeda. He was charged by the United States with treason in 2006, becoming the first person to face such US charges since the World War Two era, according to the Justice Department.
US TREATMENT OF HOSTAGES
Weinstein's wife, Elaine, said her family was devastated by his death. She criticized the US government for "inconsistent and disappointing" assistance during her husband's years in captivity. Obama said he spoke with her on Wednesday.
Like other American families whose relatives have been killed over the past year after being held hostage by militants in the Middle East, she called for a better US government policy for relaying information to hostages' families.
"We hope that my husband's death and the others who have faced similar tragedies in recent months will finally prompt the US Government to take its responsibilities seriously and establish a coordinated and consistent approach to supporting hostages and their families," she said in a statement.
US Representative John Delaney, who has helped the Weinstein family, said the United States needs to do a better job handling American hostages.
Weinstein, 73 was abducted in Lahore, Pakistan, while working for a US consulting firm. Al Qaeda had asked to trade him for members of the group held by the United States.
Weinstein was seen in videos released in May 2012 and December 2013 asking for Obama to intervene on his behalf and saying he was suffering from heart problems and asthma.
Italian media said Lo Porto, who was from Palermo, Sicily, was kidnapped three days after arriving in Pakistan to work for a German organization building houses for victims of a 2010 flood. Another man was kidnapped with him but later freed in October 2014 by German special forces.
Obama said Weinstein, who lived in Rockville, Maryland, had devoted his life to service as a member of the Peace Corps and more recently as a contractor for the US Agency for International Development in a poverty-fighting program.
CIA drone strikes in Pakistan have steeply declined from a peak of around 128 in 2010, according to the Bureau of Investigative Journalism, which tracks each strike reported by the media. There have been seven drone strikes in Pakistan so far this year, the group said.
The Pakistani government routinely protests the strikes as an infringement of national sovereignty, but many in Pakistan believe the country's powerful military at least tacitly supports the strikes.
