The United States is concerned about reports Israel used US-supplied white phosphorus munitions in an October attack in southern Lebanon, White House spokesperson John Kirby has said, Reuters reports.

“We’ve seen the reports. Certainly concerned about that. We’ll be asking questions to try to learn a little bit more,” Kirby told reporters on Air Force One.

Kirby said white phosphorus has a “legitimate military utility” for illumination and producing smoke to conceal movements.

“Obviously any time that we provide items like white phosphorous 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,” he said.