A massive sea and air search was under way on Friday for eight missing U.S. Marines off the Southern California coast after a training mishap on Thursday also resulted in one Marine dead and two others injured, the Marine Corps said in a statement.

Fifteen Marines and one Navy sailor were aboard an assault amphibious vehicle that began taking on water around 5:45 p.m. Pacific time on Thursday (0045 GMT Friday), the statement said.

One Marine was pronounced dead at Scripps Memorial Hospital La Jolla, and two others were taken to hospitals where one was listed in critical condition and the other in stable condition, the statement said.

A U.S. Navy destroyer and a Coast Guard cutter searched at sea while three Navy helicopters and a Coast Guard helicopter searched overhead, in addition to several small Navy boats, the statement said.

All the Marines involved were assigned to the 15th Marine Expeditionary Unit, which is based at Camp Pendleton, the largest Marine base on the West Coast of the United States, between Orange and San Diego counties.

Marines often practice beach assaults there using the amphibious troop transport vehicles.

“We are deeply saddened by this tragic incident. I ask that you keep our Marines, sailors and their families in your prayers as we continue our search,” Colonel Christopher Bronzi, commanding officer of the 15th Marine Expeditionary Unit, said on Twitter.

The incident occurred during what the Marine Corps called a routine training exercise near San Clemente Island. (Reuters)