One United States Marine was declared dead and five remained missing on Thursday after two American aircraft collided and crashed off the coast of Japan, officials said.

A seventh Marine was rescued and was in fair condition, the Marine Corps said in a statement.

The accident, which occurred around 2 a.m. Thursday about 200 miles off the coast of Japan, involved a KC-130 Hercules, a turboprop airplane capable of aerial refueling, and an F/A-18 Hornet, a combat jet.

The aircraft had taken off from a base at Iwakuni, in southern Japan, and were conducting a training exercise, the Marine Corps said. “Aerial refueling was a part of the training,” officials said, but “what was taking place when the mishap occurred” remained under investigation.

Ships and aircraft with the United States Seventh Fleet, joined by the Japanese Coast Guard and Maritime Self-Defense Force, were continuing to search for the missing Marines.