One American and one Italian held hostage by al Qaeda in the border region of Pakistan and Afghanistan were killed in a US counter-terrorism operation in January, the White House said on Thursday.
The operation in which American doctor Warren Weinstein and Italian Giovanni Lo Porto were killed also resulted in the death of an American al Qaeda leader, Ahmed Farouq, the White House said.
Another American al Qaeda member, Adam Gadahn, also was killed, likely in a separate operation, the White House added.
US president Barack Obama is scheduled to make a statement on the incident.
In a written statement, the White House expressed "tremendous sorrow" over the hostage deaths and said the United States had no reason to believe hostages were being held at an al Qaeda-linked compound targeted in the operation.
“Analysis of all available information has led the intelligence community to judge with high confidence that the operation accidentally killed both hostages,” the statement said.
“No words can fully express our regret over this terrible tragedy,” the statement added.
Weinstein was abducted in Lahore, Pakistan, in 2011 while working for a US consulting firm.
Al Qaeda had asked to trade him for members of the Islamist militant group being held by the US.
Weinstein was seen in videos released in May 2012 December 2013, asking for Mr Obama to intervene on his behalf and saying he was suffering from heart problems and asthma.
Italian aid worker Lo Porto has been missing in Pakistan since January 2012.
The White House did not describe the operation, but the Wall Street Journal reported that it was the first known instance in which the United States has accidentally killed a hostage in a drone strike.
The White House said Mr Obama “takes full responsibility for these operations and believes it is important to provide the American people with as much information as possible about our counter-terrorism operations, particularly when they take the lives of fellow citizens.
“The uniquely tragic nature of the operation that resulted in the deaths of two innocent hostages is something we will do our utmost to ensure is not repeated,” the statement said.
It said while the operation “was lawful and conducted consistent with our counterterrorism policies, we are conducting a thorough independent review to understand fully what happened and how we can prevent this type of tragic incident in the future.”
Italian media said Lo Porto, who was from Palermo, Sicily, was kidnapped three days after arriving in Pakistan on January 19th, 2012, to work for a German organization building houses for victims of a 2010 flood.
Another man was kidnapped with him but later separated and freed in October 2014 by German special forces.
Reuters