Two Marines aboard a helicopter were killed on Monday when the aircraft crashed during a training mission at a Southern California military base, officials said.

The two-person AH-1W Cobra helicopter went down in the southeast corner of Camp Pendleton, Marine Corps Air Station Miramar said in a statement.

Camp Pendleton is 40 miles north of San Diego.

A brush fire caused by the crash has spread to about 50 acres, and in an effort to quell the blaze officials have deployed several air tankers, the Marine Corps said.

The two Marines were pronounced dead at the scene by emergency responders, the Marine Corps said.

The cause of the crash is under investigation and the two Marines will not be identified until their families have been notified, said Marine Corps spokesman Corporal Steven Posy.