Geneva convention of 1949 states that:
Art 4. A. Prisoners of war, in the sense of the present Convention,
are persons belonging to one of the following categories, who have
fallen into the power of the enemy:
(1) Members of the armed forces of
a Party to the conflict, as well as members of militias or volunteer
corps forming part of such armed forces.
(2) Members of other militias and members of other volunteer corps,
including those of organized resistance movements, belonging to a
Party to the conflict and operating in or outside their own territory,
even if this territory is occupied, provided that such militias or
volunteer corps, including such organized resistance movements, fulfil
the following conditions: (a) that of being commanded by a person
responsible for his subordinates; (b) that of having a fixed
distinctive sign recognizable at a distance; (c) that of carrying arms
openly; (d) that of conducting their operations in accordance with the
laws and customs of war.
It seems that any such person under enemy's power has the right to be treated as a POW. On the other hand, people who are not organized, and do not "have a distinct sign" obviously could be treated as common criminals, i.e. not recognized as POWs.
It should be noted that this article only has force on the territory under control of the power under consideration.
A newer protocol of 1977 to the Geneva convention expands on this:
Art 41. Safeguard of an enemy hors de combat
A person who is recognized or who, in the circumstances, should be recognized to be hors de combat shall not be made the object of
A person is hors de combat if: (a) he is in the power of an adverse Party; (b) he clearly expresses an intention to surrender; or (c) he
has been rendered unconscious or is otherwise incapacitated by wounds
or sickness, and therefore is incapable of defending himself;
provided that in any of these cases he abstains from any hostile act
and does not attempt to escape.
A definition of a combatant is also clarified:
Art 44. Combatants and prisoners of war
Any combatant, as defined in Article 43, who falls into the power of an adverse Party shall be a prisoner of war.
While all combatants are obliged to comply with the rules of international law applicable in armed conflict, violations of these
rules shall not deprive a combatant of his right to be a combatant or,
if he falls into the power of an adverse Party, of his right to be a
prisoner of war, except as provided in paragraphs 3 and 4.
In order to promote the protection of the civilian population from the effects of hostilities, combatants are obliged to distinguish
themselves from the civilian population while they are engaged in an
attack or in a military operation preparatory to an attack.
Recognizing, however, that there are situations in armed conflicts
where, owing to the nature of the hostilities an armed combatant
cannot so distinguish himself, he shall retain his status as a
combatant, provided that, in such situations, he carries his arms
(a) during each military engagement, and (b) during such time as he is
visible to the adversary while he is engaged in a military deployment
preceding the launching of an attack in which he is to participate.
This means that a signatory to this convention has no right to just kill members of organized resistance groups who fell under their power. On the other hand, people who are suspected in terrorist activity but not belonging to the combatants are subject to civil laws installed on an occupied territory by the occupying power. They should follow article 43 of the Hague Convention of 1907:
The authority of the legitimate power having in fact passed into the hands of the occupant, the latter shall take all the measures in
his power to restore, and ensure, as far as possible, public order and
safety, while respecting, unless absolutely prevented, the laws in
force in the country.
The legal power of these international conventions is enforced in Russian Federation by the article 236 of the Criminal code "Using prohibited means and methods of waging war" with the penalty up to 20 years in prison.