Part II: Rules of Air Warfare#Chapter VI: Duties of belligerents towards neutral States and duties of neutrals towards belligerent States. - Art. 47.
Art. 47. A neutral State is bound to take the measures at its disposal to prevent aerial observations of the movements, operations and defense works of a belligerent from being made within its jurisdiction for the purpose of giving information to the other belligerent.
This rule also applies to a belligerent military aircraft on board a warship.