Related Rule
Belgium
Practice Relating to Rule 18. Assessment of the Effects of Attacks
Belgium’s Law of War Manual (1983) states:
Everything possible must be done to avoid incidental damage to civilian objects and loss of civilian life: when this damage and this loss appears to be excessive in relation to the anticipated military advantage, the attack must not take place. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 28.