allowing what? executing unlawful combatants or killing a number of civilians in retaliation for terrorattacks against soldiers?
the relevant treaty is not the geneva convention, but the Haager Landkriegsordnung from 1907 and its amendments, it protects POWs under the condition, that they were lawful combatants as defined in article 1 and it protects the civilian population as long as they dont interfere with combat. the treaty is binding international law and has since often been used in war crime trials.

though there is a special provision for militia, that is responding to invading troops within the first hours, when there has not yet been enough time to organize or to distribute military insignia, this obviously does not apply here.

edit: for those, who are unable to find this treaty, here is the result of a google-search on "Haager Landkriegsordnung" with about 3000 results: http://www.google.com/search?hl=en&i...iegsordnung%22