The UN headquarters in Gaza has reportedly been set ablaze after being hit by Israeli shells.
Witnesses and UN officials said the United Nations Relief and Work Agency building was being used as a shelter for civilians fleeing the bombardment of the Palestinian enclave when it was struck.
UN spokesman Chris Guiness said: "Our compound in Gaza has now received three hits, reportedly of white phosphorus.
"Buildings in the compound are on fire, there are loaded fuel tankers nearby. Three people have been injured.
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"It is not clear at this stage if they are UNWRA staff or some of the 700 or so civilians who have taken refuge in our compound."
The use of white phosphorous, which causes horrific burns on contact with skin, was banned as a weapon of war in civilian areas under the 1980 Geneva protocol.
