Convention (IV) respecting the Laws and Customs of War on Land and its annex: The Hague, 18 October 1907.
ANNEX TO THE CONVENTION: REGULATIONS RESPECTING THE LAWS AND CUSTOMS OF WAR ON LAND - SECTION II : HOSTILITIES - CHAPTER III : FLAGS OF TRUCE - REGULATIONS: ART. 32.
Art. 32. A person is regarded as a parlementaire who has been authorized by one of the belligerents to enter into communication with the other, and who advances bearing a white flag. He has a right to inviolability, as well as the trumpeter, bugler or drummer, the flag-bearer and interpreter who may accompany him.
https://ihl-databases.icrc.org/applic/ihl/ihl.nsf/Article.xsp?action=openDocument&;documentId=EF94FEBB12C9C2D4C12563CD005167F9

Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts (Protocol I), 8 June 1977.
Article 37 [ Link ] -- Prohibition of perfidy
1. It is prohibited to kill, injure or capture an adversary by resort to perfidy. Acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence, shall constitute perfidy. The following acts are examples of perfidy:
(a) the feigning of an intent to negotiate under a flag of truce or of a surrender;
(b) the feigning of an incapacitation by wounds or sickness;
(c) the feigning of civilian, non-combatant status; and
(d) the feigning of protected status by the use of signs, emblems or uniforms of the United Nations or of neutral or other States not Parties to the conflict.
2. Ruses of war are not prohibited. Such ruses are acts which are intended to mislead an adversary or to induce him to act recklessly but which infringe no rule of international law applicable in armed conflict and which are not perfidious because they do not invite the confidence of an adversary with respect to protection under that law. The following are examples of such ruses: the use of camouflage, decoys, mock operations and misinformation.

https://ihl-databases.icrc.org/applic/ihl/ihl.nsf/Article.xsp?action=openDocument&;documentId=3EA868BE16BCBB86C12563CD0051DB0B

Rule 58. Improper Use of the White Flag of Truce
Rule 58. The improper use of the white flag of truce is prohibited.
Practice Volume II, Chapter 18, Section B.
Summary
State practice establishes this rule as a norm of customary international law applicable in both international and non-international armed conflicts.
International armed conflicts
This is a long-standing rule of customary international law already recognized in the Lieber Code, the Brussels Declaration and the Oxford Manual.[1] It is codified in the Hague Regulations.[2] The Report of the Commission on Responsibility set up after the First World War identified the “misuse of flags” as a violation of the laws and customs of war subject to criminal prosecution.[3] This rule is contained in Additional Protocol I.[4] Under the Statute of the International Criminal Court, “making improper use of a flag of truce” constitutes a war crime in international armed conflicts when it results in death or serious personal injury.[5]
The prohibition of improper use of the white flag of truce is contained in numerous military manuals.[6] Violations of this rule constitute an offence under the legislation of many States.[7] This rule is also supported by official statements and other practice.
Non-international armed conflicts
The prohibition of improper use of the flag of truce was included in the draft of Additional Protocol II by Committee III of the Diplomatic Conference leading to the adoption of the Additional Protocols but was deleted at the last moment as part of a package aimed at the adoption of a simplified text.[9] The prohibition is contained in other instruments pertaining also to non-international armed conflicts.[10]
This rule is set forth in military manuals which are applicable in or have been applied in non-international armed conflicts.[11] Violations of this rule constitute an offence under the legislation of many States.[12]
No official contrary practice was found. There is no practice either to indicate that it would be lawful to use improperly the protection of a white flag of truce in non-international armed conflicts. Such improper use would undermine the protection to which persons advancing in good faith under a white flag are entitled (see commentary to Rule 67). It can be concluded that the general abstention from improperly using the white flag of truce in practice is based on a legitimate expectation to that effect.
Definition
Improper use refers to any use other than that for which the flag of truce was intended, namely a request to communicate, for example, in order to negotiate a cease-fire or to surrender.
https://ihl-databases.icrc.org/customary-ihl/eng/docs/v1_rul_rule58