The US attorney general has acknowledged four US citizens have been killed in drone strikes since 2009.
In a letter to the Senate judiciary committee, Eric Holder defended the targeted killing of Anwar al-Awlaki.
But he said Awlaki’s 16-year-old son as well as two other individuals were “not specifically targeted by the US”.
The disclosure comes as President Barack Obama prepares to make a speech on counter-terrorism and the drone programme on Thursday.
The president will “discuss why the use of drone strikes is necessary, legal and just, while addressing the various issues raised by our use of targeted action”, administration officials said.