President Obama has apologised after two Western hostages were inadvertently killed in a US attack on an al Qaeda base in January.
Warren Weinstein, an American doctor, and Italian Giovanni Lo Porto had been held in the border region of Afghanistan and Pakistan.
The US government has said the operation involved drone strikes.
Obama said US forces believed the target was an al Qaeda compound with no civilians present.
“This morning I has expressed my grief and condolences to the families of two hostages – one American, Warren Weinstein, and an Italian, Giovanni Lo Porto – who were tragically killed in a US counter-terrorism operation. Warren and Giovanni were aid workers in Pakistan devoted to improving the lives of the Pakistani people,” the president said at a White House briefing.
Weinstein, was was 73, was abducted in Lahore in 2011 while working for a US consulting firm. Lo Porto had been missing in Pakistan since January 2012.
The American had been seen in videos released by his captors, asking for Obama to intervene on his behalf and saying he was suffering from heart problems and asthma.
His wife, Emily, has said in a statement that the family does not fully understand all the facts surrounding his death.
Describing the two Western victims as brave men, Obama said he took full responsibility.
The president has ordered details of the operation to be declassified.
While claiming that it was lawful, Obama said an independent review was being carried out to understand what happened and prevent a repeat.
He added that the operation was thought to have killed dangerous al Qaeda members.