Two US Army helicopters crashed during a nighttime training mission in Kentucky, killing all nine soldiers on board, a general said Thursday.

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

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.

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."