Seven US troops were killed when their helicopter crashed during a transport mission in western Iraq, a defense official told AFP Friday.
The Sikorsky HH-60 Pave Hawk chopper was on a routine troop transport operation Thursday flying from Iraq to Syria when it went down, the official said.
"There were seven people aboard -- they are all believed to be dead," the official added.
The four crewmembers were all in the Air Force, but it was not immediately known which service the other troops were from, another official said.
Investigators are probing the crash's cause. A Pentagon statement said it did not appear to be a result of enemy activity.
An accompanying US helicopter reported the crash and a quick reaction force comprised of Iraqi Security Forces and US-led coalition members secured the scene.
"This tragedy reminds us of the risks our men and women face every day in service of our nations. We are thinking of the loved ones of these service members today," Brigadier General Jonathan Braga said.
"We are grateful to the Iraqi security forces for their immediate assistance in response to this tragic incident."
The identities of those killed will be released after next of kin are notified.
First introduced in the early 1980s, the aging Pave Hawk is based on the Army's Black Hawk chopper and is used often used for medical evacuation missions.
President Donald Trump took to Twitter to share his condolences.
"Our thoughts and prayers go out to the families and loved ones of the brave troops lost in the helicopter crash on the Iraq-Syria border yesterday. Their sacrifice in service to our country will never be forgotten," he wrote.
The US has operated helicopters and fixed-wing aircraft in Iraq during the war against the Islamic State group, which overran large areas north and west of Baghdad in 2014.
US forces began carrying out air strikes against IS in August 2014, a campaign that was later expanded to Syria, and has provided weapons, training and other support to forces fighting the jihadists in both countries.
Baghdad declared victory over the extremists late last year, but IS still has the ability to wage deadly violence in Iraq, including a series of attacks in the country's north that left 25 dead earlier this month.