The file photo shows that a U.S army soldier stands with his weapon at a military base in the Makhmour area near Mosul during an operation to attack Islamic State militants in Mosul, Iraq, Oct. 18, 2016. (REUTERS Photo)

WASHINGTON, Aug. 20 (Xinhua) -- U.S. Department of Defense said Monday that one U.S. soldier was killed Sunday in a helicopter crash in Iraq, adding three more were being treated for injuries.

Pentagon spokesman Robert Manning told a briefing that the incident took place during a "partnered counter terrorism mission" and that "there's no indication that the crash was caused by hostile fire."

Manning gave no other detail to the incident, including the identity of the deceased and injured soldiers, saying the investigation is ongoing.

The incident followed a similar helicopter crash in Western Iraq on March 15, in which all seven personnel aboard a U.S. military helicopter were killed.

According to public approximate numbers, the United States currently have 5,200 people operating in Iraq, 2,000 in Syria, and 14,000 in Afghanistan.