Mercinaries would be killed outright, but those casualties have no military markings - armbands or other identifiers.
Under the Law of Armed Conflict, civilians who spontaneously take up arms are not required to wear a uniform, or have identifying markers, so long as it is not "occupied" territory....those civilians would be a levee en masse, which is a lawful activity, and still warrant protections under the 6th Category of Article Four of Third Geneva Convention.