"It was a routine troop transport operation going from Iraq to Syria, nothing out of the ordinary," the official said.

"There were seven people aboard -- they are all believed to be dead."

Another official said that four of those killed in Thursday's crash belonged to the Air Force.

A Pentagon statement said the crash involved a Sikorsky HH-60 Pave Hawk and 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.

The US has operated both 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 carry out deadly violence in Iraq, including a series of attacks in the country's north that left 25 dead earlier this month.