The United Kingdom has written to the U.N. Security Council president to say its drone strike over Syria in August that killed three Islamic State fighters, including two Britons, was done in self-defense.

The letter dated Monday from Britain's U.N. ambassador invokes Article 51 of the U.N. Charter, which notes a member state's right of self-defense.

Ambassador Matthew Rycroft's letter says the target was "known to be actively engaged in planning and directing imminent armed attacks against the United Kingdom." It calls the airstrike "necessary and proportionate."

The letter echoes Prime Minister David Cameron's remarks to Parliament on Monday.

The Aug. 21 attack on a car in the Syrian city of Raqqa, an IS stronghold, represented an escalation for Britain. The U.K. hadn't yet participated in military actions in Syria.