Attacks on medical facilities are forbidden under international humanitarian law unless the facilities "are being used, outside their humanitarian function, to commit acts harmful to the enemy". Even if enemy combatants are inappropriately using the facility for shelter, the rule of proportionality usually forbids such attacks because of the high potential for civilian casualties.[27] Human Rights Watch said the laws of war require the attacking force to issue a warning, and wait a reasonable time for a response, before attacking a medical unit being misused by combatants.[27][35]