TOKYO - An F/A-18 fighter jet and a KC-130 tanker plane belonging to the US Marines collided and crashed off Japan's western coast early Thursday morning, with five of the seven crew members still missing, the Japanese Defense Ministry said.

One of those rescued by the Japanese Self-Defense Forces (SDF) was in the F/A-18 fighter jet and was taken to the Marine Corps Air Station Iwakuni in western Japan, where he is said to be in a stable condition, Japanese officials said.

The other rescued crew member is assumed to have been taken to the same facility, although his condition had not yet been made clear.

According to officials, there were two crew members aboard the fighter jet and five crew members aboard the tanker plane when the collision occurred at around 1:40 am local time.

The ministry said the planes, based at the Marine Corps Air Station Iwakuni in western Japan, were conducting a routine training operation when the collision occurred around 100 km south of Cape Muroto in Kochi Prefecture.

There is speculation that the F/A-18 fighter jet was refueling mid-air when the collision with the KC-130 tanker took place, although there has been no official confirmation of this as yet.

The F/A-18 aircraft, a multirole combat jet, is designed as both a fighter and attack aircraft and known colloquially as the "Hornet."

The KC-130 tanker plane is designed to refuel both fixed-wing, tilt-rotor and rotary-wing aircraft using the probe and drogue technique, aviation experts explained.

Mid-air refueling can be a tricky process especially at night and in inclement weather, they added of the matter.

A JSDF search and rescue aircraft was quickly deployed to help with rescue efforts, while other SDF aircraft and vessels are also searching the area for survivors, the Marines said.

The SDF have deployed a total of 10 aircraft, while the Japan Coast Guard has dispatched six patrol vessels to conduct the ongoing search and rescue operations, officials from the government here said.

The Japanese Defense Ministry is currently trying to ascertain more details from the US forces in Japan about the accident, but has confirmed that no civilian ships have reported being affected by the incident.

US Forces Japan said in a statement that it was thankful for the quick response of the Japan Maritime Self-Defense Force "as they immediately responded in the search and rescue operation."

"The circumstances of the mishap are currently under investigation," it added.

The latest US aircraft mishap comes on the heels of another F/A-18 fighter aircraft, belonging to the USS Ronald Reagan nuclear carrier, crashing into the Pacific Ocean in waters southwest of Kita Daitojima Island around 290 km from Okinawa, owing to mechanical issues, on Nov. 12.

On Oct 19, a US Navy MH-60 Seahawk helicopter crashed on the deck of the USS Ronald Reagan during routine operations.

The crashes occurring within a month of each other and related to the same aircraft carrier, sparked a great deal of concern from the Japanese government, who said it will strongly request information about the accident amid safety concerns and local citizens' fears.

Chief Cabinet Secretary Yoshihide Suga said at the time that accidents "involving US military cause immense worry to people in the area and should not occur."

Japan's top government spokesperson at the time vowed to "strongly request that the United States provide us with information and ensure absolute safety management."