The International Red Cross said Tuesday that Israel has fired white phosphorus shells in its offensive in the Gaza Strip, but has no evidence to suggest it is being used improperly or illegally. The comments came after a human rights organization accused the Jewish state of using the incendiary agent, which ignites when it strikes the skin and burns straight through or until it is cut off from oxygen. It can cause horrific injuries. The International Committee of the Red Cross urged Israel to exercise "extreme caution" in using the incendiary agent, which is used to illuminate targets at night or create a smoke screen for day attacks, said Peter Herby, the head of the organization's mines-arms unit. "In some of the strikes in Gaza it's pretty clear that phosphorus was used," Herby told The Associated Press. "But it's not very unusual to use phosphorus to create smoke or illuminate a target. We have no evidence to suggest it's being used in any other way." In response, the IDF said Tuesday that it "wishes to reiterate that it uses weapons in compliance with international law, while strictly observing that they be used in accordance with the type of combat and its characteristics." Herby said that using phosphorus to illuminate a target or create smoke is legitimate under international law, and that there was no evidence the Jewish state was intentionally using phosphorus in a questionable way, such as burning down buildings or knowingly putting civilians at risk. However, Herby said evidence is still limited because of the difficulties of gaining access to Gaza, where Palestinian health officials say more than 900 people have been killed and 4,250 wounded since Israel launched its offensive late last month. The operation aims to halt years of Palestinian rocket attacks over the border. Human Rights Watch accused Israel of firing phosphorous shells and warned of the possibilities of extreme fire and civilian injuries. The chemical was suspected in the cases of 10 burn victims who had skin peeling off their faces and bodies. White phosphorus is not considered a chemical weapon.