Related Rule
France
Practice Relating to Rule 20. Advance Warning
France’s LOAC Summary Note (1992) states: “If the military mission allows for it, appropriate warning must be given to the civilian population to give it time to seek shelter.” 
France, Fiche de Synthèse sur les Règles Applicables dans les Conflits Armés, Note No. 432/DEF/EMA/OL.2/NP, Général de Corps d’Armée Voinot (pour l’Amiral Lanxade, Chef d’Etat-major des Armées), 1992, § 1.4.
The instructions given to the French armed forces for the conduct of Opération Mistral, simulating a military operation under the right of self-defence or a mandate of the UN Security Council, state that “an individual warning must be given prior to any attack against a civilian ship or aircraft approaching or entering an exclusive or similar zone, to the extent that the tactical situation permits”. 
France, Etat-major de la Force d’Action Rapide, Ordres pour l’Opération Mistral, 1995, Section 5.