Related Rule
Germany
Practice Relating to Rule 64. Conclusion of an Agreement to Suspend Combat with the Intention of Attacking by Surprise the Adversary Relying on It
Germany’s Military Manual (1992) gives as an example of an act of perfidy the conclusion of a “humanitarian agreement to suspend combat with the intention of attacking by surprise the enemy relying on it”. 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 472.
The manual also states: “During an armistice, it is … definitely forbidden to move the forces in contact with the enemy forward or to employ reconnaissance patrols.” 
Germany, Humanitarian Law in Armed Conflicts – Manual, DSK VV207320067, edited by The Federal Ministry of Defence of the Federal Republic of Germany, VR II 3, August 1992, English translation of ZDv 15/2, Humanitäres Völkerrecht in bewaffneten KonfliktenHandbuch, August 1992, § 237.