Two US marines have been found and five others remain unaccounted for after two US Marine Corps aircraft collided off the coast of western Japan early Thursday.

"One of the personnel involved in the mishap is being evaluated by competent medical authorities... The search and rescue operations continue for the remaining six Marines," the Marines tweeted in the morning.

The marine rescued by the Japanese Navy "is in a stable condition," Japanese government spokesman Yasutoshi Nishimura told a news conference.

The Japanese Navy have also found a second marine off the south-western island of Shikoku after the crash, Japan’s Defence Ministry said.

However, the person’s condition is still unknown, a spokesman for the ministry told dpa.

The warplanes involved, a KC-130 refueling aircraft carrying five crew members and an F/A-18 jet with two on board, had taken off from Marine Corps Air Station Iwakuni, Nishimura said.

The collision occurred about 100 kilometres south of Cape Muroto on Hikoku, the Defence Ministry in Tokyo said.

Japanese Foreign Ministry official Kazuhiro Suzuki urged Jerry Martinez, commander of US Forces Japan, to immediately provide information related to the accident, conduct an investigation to determine its cause and prevent a recurrence.

The planes were "conducting regularly scheduled training when the mishap occurred," according to a news release issued by the Marine base at Camp Butler in Okinawa.

Japanese search and rescue aircraft immediately responded to aid in the recovery, the Marines said, adding that the accident occurred at about 2 am Thursday (1700 GMT Wednesday).

"The circumstances of the mishap are currently under investigation," the news release said, offering no additional information.

There have been a series of accidents in and around Japan involving the US military. Thursday's crash comes less than a month after a US fighter jet went down off the southern island of Okinawa in mid-November and its two crew members were rescued.

Twelve sailors were injured in October after a US Navy MH-60 Seahawk helicopter crashed on the deck of the carrier in the Philippine Sea.

In October 2017, a US CH-53 helicopter went down near a residential area in northern Okinawa. None of its crew members was injured.