The use of white phosphorus is not in and of itself a war crime, and is generally considered acceptable as a means of obscuring troop movements or illuminating areas. Its use in civilian areas however, even if not directed at the civilian population, is banned under the Geneva Convention on Certain Conventional Weapons. Preliminary investigations show indisputable evidence that Israel used white phosphorus in some of the most densely populated portions of Gaza, and still burning fragments were found after the war ended wedged into civilian buildings.
The Israeli military officially denied using such munitions during the war, though they eventually conceded to it. Their official story now is that the use was not illegal and that Hamas was the one committing war crimes by provoking such attacks. The treaty prohibits the use of such weapons against military targets in civilian regions however, and makes no exception allowing the nation violating it to transfer blame to others in case they really wanted to hit those targets.