Two US Army Blackhawk helicopters crashed in Kentucky during a training mission Wednesday night, officials said.
"The status of the crew members are unknown at this time," army base Fort Campbell said in a statement early Thursday.
"The command is currently focused on caring for the servicemembers and their families."
The helicopters, from the 101st Airborne Division, crashed at around 10:00 pm Wednesday in Kentucky's Trigg county, northwest of Fort Campbell.
Kentucky Governor Andy Beshear said fatalities were expected.
"We've got some tough news out of Fort Campbell, with early reports of a helicopter crash and fatalities are expected," he said on Twitter.
He said Kentucky police and emergency management were responding.
"Please pray for all those affected," Beshear tweeted.
Fort Campbell said the incident was under investigation.
"More information will be released as it becomes available," it said.