The US admitted a counter-terrorism operation conducted in Afghanistan in January killed two hostages, who were detained by al-Qaeda.
Speaking at the White House, President Barack Obama said it was a painful loss he deeply regretted.
The president said the US had launched the raid in the belief the target was an al-Qaeda compound with no civilians present and was not aware of the presence of aid workers Warren Weinstein, an American, and Giovanni Lo Porto, an Italian.
Two militants, Ahmed Farouq, an al-Qaeda leader, and Adam Gadahn, an American member of al-Qaeda are also thought to have died in the attack.
It's a "bitter truth in the fog of war" that mistakes occur, the president said, but said what sets America apart is that it faces up squarely to its mistakes.
President Obama said as commander-in-chief he took "full responsibility" for the operation and ordered a review.