Amnesty International has accused Israel of using white phosphorus in civilian areas of the Gaza Strip.
The substance can kill or cause serious injuries by burning through skin, and it is banned near civilians. Armies may use it to create smoke screens.
The UN and Human Rights Watch have already accused the Israeli army of firing white phosphorus shells in Gaza.
Israel has said all the weapons it used in its three-week offensive in Gaza were permitted by international law.
Amnesty said a fact-finding team found "indisputable evidence of the widespread use of white phosphorus" in crowded residential areas of Gaza City and elsewhere in the territory.
"Yesterday, we saw streets and alleyways littered with evidence of the use of white phosphorus, including still-burning wedges and the remnants of the shells and canisters fired by the Israeli army," said Christopher Cobb-Smith, a weapons expert with the Amnesty team in Gaza.
'Unusual burns'
The Amnesty group said one of the places worst-affected by white phosphorus was the UN Relief and Works Agency (Unrwa) compound in Gaza City.
WHITE PHOSPHORUS CLAIMS
Israeli shell fire set the compound on fire on 15 January, burning stocks of food and other humanitarian supplies in a warehouse and coming close to stocks of fuel.
The head of Unrwa in Gaza, John Ging, said at the time that "three rounds that emitted phosphorus" hit the compound.
The Israeli military said it had come under fire from Palestinian fighters inside the compound and had fired back.
Human Rights Watch also said it observed "dozens and dozens" of white phosphorus shells being fired by Israel at the Gaza Strip.
Palestinian medical officials have also said that large numbers of casualties with unusual burns, possibly consistent with exposure to white phosphorus, had been treated at Gaza Strip hospitals.
White phosphorus sticks to human skin and will burn right through to the bone, causing death or leaving survivors with painful wounds which are slow to heal. Its ingestion or inhalation can also be fatal.
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