Nine people are dead after two US Army helicopters crashed in southwestern Kentucky during a routine training mission.

The two HH-60 Black Hawk helicopters, part of the 101st Airborne Division, crashed around 10pm Wednesday in Trigg County, Kentucky, according to a statement from Fort Campbell.

Debris from a helicopter crash is pictured at the crash site in Trigg County, Kentucky. - Reuters

The 101st Airborne confirmed the crash about 30 miles (48 kilometres) northwest of Fort Campbell, saying on Twitter it resulted in "several casualties" but did not specify whether those were injuries or deaths.

"We’ve got some tough news out of Fort Campbell, with early reports of a helicopter crash, and fatalities are expected," governor Andy Beshear said in a post on Twitter, adding that local authorities and emergency services were responding to the accident.

Brig. Gen. John Lubas, the 101st Airborne deputy commander, said it is unclear what caused the crash.

“This was a training progression, and specifically they were flying a multi-ship formation, two ships, under night vision goggles at night," Lubas said. He said officials believe the accident occurred when "they were doing flying, not deliberate medical evacuation drills.”

The helicopters have something similar to the black boxes on passenger planes, which records the performance of aircrafts in flight and are used by investigators to analyze crashes.

“We’re hopeful that will provide quite a bit of information of what occurred,” Lubas said.

A view of the site after helicopters crashed in Trigg County, Kentucky. - Reuters

"The crash occurred in a field, some wooded area," Kentucky State Police Trooper Sarah Burgess said at a news briefing. "At this time, there are no reports of residence damage."

Fort Campbell is located near the Tennessee border, about 60 miles (97 kilometres) northwest of Nashville, and the crash occurred in the Trigg County, Kentucky, community of Cadiz.

Nick Tomaszewski, who lives about a mile from where the crash occurred, said he saw two helicopters flying over his house moments before the crash.

"For whatever reason last night my wife and I were sitting there looking out on the back deck and I said, 'Wow, those two helicopters look low and they look kind of close to one another tonight.'" 

The helicopters flew over and looped back around and moments later "we saw what looked like a firework went off in the sky."

"All of the lights in their helicopter went out. It was like they just poofed ... and then we saw a huge glow like a fireball," Tomaszewski said.

Brig. Gen. John Lubas address the press in regards to the Black Hawk helicopter crash that occurred early Thursday, March 30, outside of Fort Campbell, Kentucky. - AP

Flyovers for training exercises happen almost daily and the helicopters typically fly low but not so close together, he said.

"There were two back to back. We typically see one and then see another one a few minutes later, and we just saw two of them flying together last night," he said.

Members of the Kentucky Senate stood for a moment of silence Thursday morning in honour of the crash victims.

"We do not know the extent of what has gone on, but I understand it is bad and there has been a substantial loss of life of our military," Senate President Robert Stivers told the sombre chamber.

Last month, two Tennessee National Guard pilots were killed when their Black Hawk helicopter crashed along an Alabama highway during a training exercise.

US Defense Secretary Lloyd Austin offered his condolences to the families of those killed.

“My heart goes out to the families of these service members and to the members of the 101st Airborne Division who bravely and proudly serve our country each and every day," Lloyd said.