One unconscious person was found in the sea on Wednesday and later declared dead after the aircraft crashed off the island of Yakushima, according to the Japanese coastguard.

US Air Force Special Operations Command said eight crew had been aboard the CV-22B Osprey as it performed "a routine training mission" out of Yokota Air Base in Japan.

"The cause of the mishap is currently unknown," it said in a statement Wednesday, with emergency personnel "on scene conducting search and rescue operations."

An emergency management official in the Kagoshima region where the crash took place said police had received information that the aircraft had been "spewing fire from a left engine".

Photos released by the coastguard showed what appeared to be an overturned yellow life raft and other debris in the water off Yakushima, which lies south of Japan's southernmost main island of Kyushu.

A Japanese coastguard spokesman told AFP on Thursday that the search operation had continued through the night and involved six patrol ships and two aircraft.

Police and local rescuers were also involved, and the coastguard said it would use special sonar devices to scan the sea floor.

The coastguard had initially said eight crew were on board before revising the number down to six and then back to eight.

- String of crashes -

The Osprey, developed by Bell Helicopters and Boeing and which can operate like a helicopter or a fixed-wing plane, has suffered a string of fatal crashes over the years.

In August, a crash in northern Australia killed three US marines among the 23 on board.

The Boeing MV-22B Osprey crashed on Melville Island, north of Darwin during a military exercise for locally based troops. At the time the cause was unclear.

Four US Marines were killed in Norway last year when their MV-22B Osprey aircraft went down during NATO training exercises.

Three Marines were killed in 2017 when an Osprey crashed after clipping the back of a transport ship while trying to land at sea off Australia's north coast.

And 19 Marines died in 2000 when their Osprey crashed during drills in Arizona.

- Grounded -

In 2016, an MV-22 Osprey crash-landed off Okinawa, prompting the US Marines to temporarily ground the aircraft in Japan after the accident sparked anger among locals.

A defence ministry official told the ruling Liberal Democratic Party (LDP) in a meeting on Thursday that it plans to ask the US to suspend Osprey flights, an LDP official told AFP.

A day before, Prime Minister Fumio Kishida had suggested his government would not immediately make that request or consider doing so for Ospreys used by the Japanese military, the Kyodo news service reported.

"It is an issue we should think about after confirming what has actually happened," the premier told reporters, according to Kyodo.

The US military has around 54,000 personnel in Japan.