"In war you are allowed to attack your enemy," a US embassy spokesman in London said.
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..."
A clear implication of this, however, which the Obama administration hasn't always wanted to own, is that the effort to dismantle and disrupt terrorist networks is indeed a war.