The helicopters belonged to the 101st Airborne Division, whose spokesman Lieutenant Colonel Anthony Hoefler said the nine soldiers had died.

The crash occurred at around 10:00 pm (0300 GMT) Wednesday in Kentucky's Trigg county, northwest of Fort Campbell, the base said.

"The crewmembers were flying two HH60 Blackhawk helicopters during a routine training mission when the incident occurred," it said in a statement, adding that the incident is under investigation.

Kentucky Governor Andy Beshear tweeted that he would be "traveling to Fort Campbell to support our troops and their families after last night's tragic incident."

Beshear had earlier said on Twitter that Kentucky police and emergency management were responding, saying: "Please pray for all those affected."

Fort Campbell is home to the 101st Airborne Division, the US Army's only air assault division.

Nicknamed the "Screaming Eagles," the division 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.