US State Department spokesperson Matt Miller on Monday reiterated concern about a report that Israel has used white phosphorous in southern Lebanon.

"Obviously there is a legitimate military use for white phosphorus but that does not include using them on civilians," he said at a news briefing.

"Anytime that we provide items like white phosphorus or really anything to another military, we do it with the expectation that it will be used for legitimate purposes and in fully keeping with humanitarian law and law of armed conflict," he said. 

Miller said that the US is looking into the matter and "for additional information."

CNN is unable to confirm the use of phosphorous munitions. In a previous statement to CNN, the Israel Defense Forces (IDF) said that it uses “only legal weapons and ammunition.”

Israel's history with white phosphorus: Israel previously faced widespread criticism for firing white phosphorus shells over densely populated areas during a Gaza offensive that began in late 2008. HRW said in a 2009 report that Israel's white phosphorus munitions had killed and injured civilians and damaged civilian structures, including a school, a market, a humanitarian aid warehouse, and a hospital. HRW claimed that Israel's use of the weapons in crowded neighborhoods "violated international humanitarian law (the laws of war), which requires taking all feasible precautions to avoid civilian harm and prohibits indiscriminate attacks."

In response, Israel pledged to limit the use of white phosphorus and make greater efforts to protect civilians during conflicts. Still, the government said that it had used white phosphorus lawfully.

Claim of recent use: In October, Human Rights Watch accused Israeli forces of using white phosphorus in Gaza and Lebanon.

According to the HRW report, the rights group said it verified one video taken on October 10 in Lebanon and another video in Gaza on October 11 that it claimed shows "multiple airbursts of artillery-fired white phosphorus over the Gaza City port and two rural locations along the Israel-Lebanon border."

Israel denied the claims by Human Rights Watch.

CNN's Mohammed Tawfeeq contributed to this report.