Human Rights Watch says use of the chemical in populated areas violates legal obligation to avoid civilian casualties.
Human Rights Watch said it verified footage taken in Lebanon and Gaza on Tuesday and Wednesday, respectively, showing multiple uses of artillery-fired white phosphorus over the Gaza City port and two rural locations along the Israel-Lebanon border.
Human Rights Watch also interviewed two people who described an attack in Gaza, the rights group said.
HRW said the use of white phosphorus in densely populated areas violates Israel’s obligation under international law to take all feasible precautions to avoid harm to civilians.
“Any time that white phosphorus is used in crowded civilian areas, it poses a high risk of excruciating burns and lifelong suffering,” Lama Fakih, HRW’s director for the Middle East and North Africa, said in a statement on Wednesday.
“White phosphorus is unlawfully indiscriminate when airburst in populated urban areas, where it can burn down houses and cause egregious harm to civilians.”
Israel should ban all use of “airburst” white phosphorus munitions in populated areas without exception, the rights group said, noting the availability of non-lethal alternatives.
White phosphorus, which can cause bone-deep burns resulting in death or lifelong injuries, can be used to generate smokescreens, mark targets and incinerate enemy targets.
While the use of the substance in warzones is not banned outright under international law, use near civilians is prohibited under the United Nations Convention on Conventional Weapons, to which Israel is not a signatory.
Israel’s defence ministry did not immediately respond to Al Jazeera’s request for comment.
But Israel’s military was quoted by the Reuters news agency as saying it was “currently not aware of the use of weapons containing white phosphorus in Gaza”.
Officials did not provide comment on the alleged use of the chemical in Lebanon, Reuters said.
Israel used white phosphorus extensively during its bombardment of Gaza in 2009, drawing widespread condemnation.
In 2013, the Israeli military said it would no longer use white phosphorus in populated areas except in exceptional circumstances, following a petition to Israel’s High Court.
Israel has pledged to eradicate Hamas from Gaza following the armed group’s deadliest attack inside the country in decades over the weekend.
Israeli air strikes on Gaza have killed more than 1,500 Palestinians and wounded more than 6,600 others, according to officials in the enclave.
More than 1,300 people were killed and 3,000 injured in Hamas’s multi-pronged attack on southern Israeli towns on Saturday, according to Israeli officials.