ICAO Annex 2
3.2 Avoidance of collisions
3.2.2 Right-of-way

3.2.2.3 Converging. When two aircraft are converging at approximately the same level, the aircraft that has the other on its right shall give way, except as follows:

a) power-driven heavier-than-air aircraft shall give way to airships, gliders and balloons;
b) airships shall give way to gliders and balloons;
c) gliders shall give way to balloons;
d) power-driven aircraft shall give way to aircraft which are seen to be towing other aircraft or objects.

• Oder of priority: Ballons → Gliders → Airships → Towing aircraft → Power-driven heavier-than-air aircraft