A rescue mission is underway for missing crew members following an aerial disaster involving two US Marine Corps fighter jets that collided mid-air off the coast of Japan, the Japan Ministry of Defense and US Marine Corps revealed Thursday.

According to the Marines, the crash happened at 1:42 a.m. local time. A KC-130 Hercules plane and a F/A-18 Hornet fighter jet collided during a routine exercise as part of the crew’s training. The former’s primary mission was airborne refueling.

“The aircraft were conducting routine training, and aerial refueling was a part of the training, As to what was taking place when the mishap occurred, that is under investigation,” the Marine Corps said in a statement.

The incident which happened during the early hours of Thursday left one Marine dead. Five more are believed to be missing, while the seventh member has been rescued safely.

CNN reports that the rescued Marine is in “fair condition,” while the deceased member was found by a Japanese ship and was transferred to Japan for medical care. He was later declared dead.

Japan has already dispatched 10 aircrafts and three ships to help with the search and rescue of the five missing people. The U.S. has also sent a Navy P-8A Maritime Patrol and Reconnaissance Aircraft to help with the mission.

The planes involved in the accident took off from Iwakuni, Japan — one of the biggest U.S. air bases in East Asia, the Washington Post noted. The base is home to 15,000 staff from both the U.S. and Japanese defense forces.

The incident follows a series of crashes in the coasts of Asian countries. Just last month, an F/A-18 Super Hornet fighter jet crashed in Okinawa. A few more have crashed the past months, including one which plummeted in the Philippine Sea in October.