(Photo: Hellfire ... artillery shells explode above Gaza City, Gaza split by Israeli invasion)
It appears that Israel is firing white phosphorus ("Willy Pete" or WP) rounds into Gaza. Hubris Sonic identifies the rounds illustrated as WP.
(Photo: Israeli artillery shells explode with a chemical agent designed to create smokescreen for ground forces (Jack Guez/AFP/Getty Images), Israel splits Gaza in three as soldiers battle Hamas)
White phosphorus is a chemical round with multiple uses. It can be used as a smoke round to mark locations. It can be used as an incendiary round against materiel and structures. Both of these uses are legal. It is also used as an incendiary against personnel, which use is of questionable legality.
WP burns in the presence of oxygen, and will burn even inside the human body. It must be smothered with water or oil, and is toxic while burning in the body, if ingested, and if inhaled.
(Photo: Artillery shells explode above Gaza City, Getty Images)
According to US military doctrine, "It is against the law of land warfare to employ WP against personnel targets":
(4) Burster Type White phosphorus (WP M110A2) rounds burn with intense heat and emit dense white smoke. They may be used as the initial rounds in the smokescreen to rapidly create smoke or against material targets, such as Class V sites or logistic sites. It is against the law of land warfare to employ WP against personnel targets.This Guardian story indicates that WP is illegal to use against civilians, and points out that WP, because it is a chemical weapon, is a WMD. The US previously used WP in the battle for Fallujah in Iraq. American doctrine lumps nuclear, biological, and chemical weapons together as WMD and the US reserves the right to respond to any WMD attack with nuclear weapons.
Other stories which discuss the invasion, use of cluster munitions, etc.:
Pacific Free Press
Update: Stirling Newberry has a great story on it