Seven US Marines have been killed after two helicopters collided during a night training exercise near the California-Arizona border, the Marine Corps has said.
An AH-1W 'Cobra' and UH-1 'Huey' aircraft hit each other in mid-air north of Yuma, Arizona. There were no survivors.
An investigation has been launched to find out what caused the accident, which happened at 8pm local time on Wednesday.
The men were based at Camp Pendleton north of San Diego, California.
A Marine Corps spokeswoman, Lt Maureen Dooley, said it would be at least 24 hours before their names are released.
She said helicopter teams train in the area because the conditions are similar to those in Afghanistan.
When asked whether the crash was a result of technical or pilot error, she said she unable to comment.
Captain Staci Reidinger, director of public affairs at Marine Corps Air Station Yuma, said the incident was not in a built-up area.
The senior US senator from Arizona, John McCain, expressed his condolences in a statement after the crash.
Mr McCain, a former Navy pilot who was taken prisoner during the Vietnam War, said "My thoughts and prayers are with the families of the seven Marines killed in the helicopter collision last night near Yuma, Arizona.
"This tragedy is a grave reminder of the sacrifices the men and women of our armed forces make to keep us safe - both in training here at home and in combat abroad."
There have been a number of accidents in the past year involving Marine Corps training in Southern California.
In September, a helicopter went down at Camp Pendleton, killing the two Marines on board.
In August, two Marines were ejected from their fighter jet as it plunged towards the Pacific Ocean.
They spent four hours in the sea before they were rescued. Both had broken bones but survived.
In July, a Marine was killed when his helicopter went down in a remote section of Camp Pendleton.
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