The crash -- which involved variants of the Black Hawk designed for medical evacuation -- was the deadliest such incident in more than eight years.

The aircraft were flying in formation with pilots using night vision goggles during a routine training exercise, and were steered to land in an open field across from a residential area, avoiding deaths or injuries on the ground, Brigadier General John Lubas told a news conference.

The crash "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, Lubas said.

Five soldiers were on one helicopter and four on another, with the military still working to notify all the families of those killed.

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.

"Freedom relies on those who are willing to serve, some of which pay the ultimate price," the governor told the news conference.

US Defense Secretary Lloyd Austin said in a statement he is "saddened by this tragic loss," while White House Press Secretary Karine Jean-Pierre told journalists that "we stand with all who are grieving in the wake of this terrible, truly terrible, accident."

- 'Thoroughly investigated' -

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.

The last time nine or more people died in a training-related incident involving a helicopter was on March 10, 2015, when a Louisiana National Guard Black Hawk crashed during a nighttime training mission off the Florida coast, killing 11 people, US Army Combat Readiness Center spokesman Jimmie Cummings told AFP.

"Each accident is thoroughly investigated, and lessons learned are applied to improve safety standards and processes," Cummings said.

There have been multiple other 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.