A more thorough explanation of the legal basis was given last year by Harold Hongju Koh, legal adviser at the US state department. He told a meeting of the American Society of International Law: "Some have argued that the use of lethal force against specific individuals fails to provide adequate process and thus constitutes unlawful extrajudicial killing.
But a state that is engaged in an armed conflict or in legitimate self-defence is not required to provide targets with legal process before the state may use lethal force.
"The principles of distinction and proportionality that the US applies are
implemented rigorously throughout the planning and execution of lethal operations to ensure that such operations are conducted in accordance with all applicable law."
He added: "Some have argued that our targeting practices violate domestic law, in particular, the longstanding domestic ban on assassinations.
But under domestic law, the use of lawful weapons systems - consistent with the applicable laws of war - for precision targeting of specific high-level belligerent leaders when acting in self-defence or during an armed conflict is not unlawful, and hence does not constitute 'assassination'."
Osama bin Laden: US responds to questions about killing's legality | World news | The Guardian