WASHINGTON (AFP) - Seven US Marines and four aircrew were missing and presumed dead on Wednesday after their army helicopter crashed during a night training exercise in Florida, military officials said.
The UH-60 Black Hawk helicopter was reported missing Tuesday around 8.30pm (0030 GMT Wednesday) near Eglin Air Force Base.
The area was under heavy fog but it was not immediately clear if that played a part in the accident.
Base spokesman Andy Bourland said rescue teams located debris from the aircraft at around 2am and an investigation was ongoing.
"The thoughts and prayers of everybody here at the White House are with the families of those who were killed in this that occurred apparently overnight," White House spokesman Josh Earnest said.
President Barack Obama had expressed his condolences in phone calls with the military commanders affected.
"The President reassured the commanders of the nation's deep appreciation for the many sacrifices that our men and women in uniform and their families make to protect and defend our country," Earnest added.
A second Black Hawk returned safely to base.
The choppers, assigned to the 1-244th Assault Helicopter Battalion in Hammond, Louisiana, were participating in what the military called a routine training mission with the Marine Special Operations Regiment from Camp Lejeune, North Carolina.
Major General Glenn Curtis said: "As we speak, there's an ongoing search and rescue operation to search for that aircraft and search for our service members that were on it, and that's as much as I know at this point."
Curtis, of the Louisiana National Guard, added: "At this hour our priorities, my priorities, are search and rescue for our soldiers and the Marines, and secondly is to take care of our families. And we are doing both of those. This will remain a search and rescue operation until further notice."
Defence Secretary Ashton Carter told lawmakers during a hearing: "My thoughts and prayers are with their families as the search and rescue continues."