Lebanon accused Israel of repeatedly using the incendiary weapon in October, while the Washington Post on Monday said analysis of shell fragments from one attack showed the rounds were US-made.

"We've seen the reports, we're certainly concerned about that. We'll be asking questions to try to learn a little bit more," said National Security Council spokesman John Kirby.

The use of white phosphorus as a chemical weapon is prohibited under international law, but it is allowed for illuminating battlefields and can be used as a smokescreen.

"Obviously, any time that we provide items like white phosphorus to another military, it is with the full expectation that it will be used in keeping with those legitimate purposes and in keeping with the law of armed conflict," said Kirby.

The Washington Post said nine civilians were injured in an Israeli attack in southern Lebanon using what appeared to be US-supplied white phosphorus.

A journalist working for the newspaper found remnants of three artillery rounds with serial numbers showing they were made in the United States in 1989 and 1992, it said.

Lebanon said on October 31 that Israel's use of the weapon in "repeated" attacks -- amid tensions following the October 7 attacks on Israel by the Hamas movement in Gaza -- had burned 40,000 olive trees.

The United States has strongly backed Israel but has been trying to keep the conflict from spreading throughout the region, especially to Lebanon where the Iran-backed Hezbollah group is based.

Kirby said it was "also in the context of that, that we're concerned about these reports" on the use of white phosphorus.

State Department spokesperson Matthew Miller added that the United States was "looking for additional information."

"Anytime that we saw white phosphorus being used in a way that would harm civilians, of course, it's something that we would be concerned about," Miller added.