The helicopters -- variants of the Black Hawk designed for medical evacuation -- were taking part in a routine training mission the night before when the crash occurred, Brigadier General John Lubas told a news conference.

It "resulted in the death of all nine service members aboard the aircraft," all of them members of the 101st Airborne Division, which is based at Fort Campbell in Kentucky, he said.

Five soldiers were on one helicopter and four on another, Lubas said, noting that the military is still working to notify all the families of those killed.

The helicopters -- which were flying in formation with the pilots using night vision goggles -- were able to land in an open field across from a residential area, avoiding deaths or injuries on the ground, he said.

With an investigative team heading to Fort Campbell from the base where US Army Aviation is headquartered in Alabama, it was still unknown whether the two helicopters collided.

"We have a safety team coming... from Fort Rucker, Alabama who specialize in aircraft safety and specifically these investigations," Lubas said.

Kentucky Governor Andy Beshear mourned the lost soldiers and praised those who responded to the crash.

"We are blessed to live in the freest country in the history of planet Earth. But we must remember that that freedom relies on those who are willing to serve, some of which pay the ultimate price," the governor told the news conference.

- 'Just disappeared' -

US Defense Secretary Lloyd Austin also offered his condolences, saying in a statement that his "heart goes out to the families of these servicemembers."

"I'm saddened by this tragic loss, and I am working with army leadership to make sure our troops and their families receive the care that they need in the wake of this accident," he said.

MSNBC quoted a local resident who witnessed the crash.

"Two helicopters just disappeared out of the sky. There was a large flash," Nick Tomaszewski said, adding that another helicopter circled the area for about 30 minutes before ambulances arrived.

There have been multiple crashes of US military aircraft in recent years, including another involving a Black Hawk that killed two Tennessee National Guardsmen during a training flight in Alabama in February.

Four US Marines were killed during NATO exercises in Norway last year when their V-22B Osprey aircraft went down, possibly after hitting a mountain, investigators said.

And two US Navy pilots were rescued after their T-45C Goshawk jet crashed during a training exercise in a residential neighborhood near Fort Worth, Texas in 2021. The pilots ejected before the plane went down.

The 101st Airborne Division is the US Army's only air assault division. Nicknamed the "Screaming Eagles," it was activated in August 1942 and gained renown during World War II in the D-Day landings and the Battle of the Bulge.

More recently the division has seen action in Iraq and Afghanistan.