The Biden administration said on Monday that it is looking into reports by Amnesty International and The Washington Post that Israel used white phosphorus supplied by the US in violation of international law.

National Security Council spokesperson John Kirby said in an exchange with reporters that the Biden administration was “certainly concerned” about reports of Israel illegally using US-supplied white phosphorus in October in Lebanon. “We’ll be asking questions to try and learn a little bit more,” Kirby said.

White phosphorus is an incendiary, toxic substance used to create light and smoke screens during combat. Using it isn’t illegal, but deploying it deliberately against civilians or in a civilian setting violates the laws of war.

White phosphorous can cause eye and respiratory injuries and extremely painful and sometimes fatal burns, according to the WHO. It can also impact the cardiovascular and central nervous systems, potentially leading to loss of consciousness or coma. Israel’s military said it complies with international law and denied using the weapon illegally.

Kirby’s comments came after the Post on Monday reported on earlier claims from Amnesty International that Israel illegally deployed white phosphorus between October 10 and October 16 in civilian areas of southern Lebanon, along the border with Israel.

The Post said that, in its own investigation, its journalists found remnants of artillery rounds fired into Dheira, Lebanon, near the border of Israel, which residents said destroyed homes and injured people in the region, leaving at least 3 people hospitalised.