One unconscious person had been found in the sea near the scene of the crash off the island of Yakushima but was later "confirmed dead in hospital", the coastguard said in a statement.

It also revised downwards an earlier statement to say that six crew had been on board instead of eight.

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".

The US military, which has around 54,000 personnel in Japan, has yet to comment publicly on the crash.

The coastguard said it has mobilised patrol ships and aircraft around Yakushima, which lies south of Japan's southernmost main island of Kyushu.

Japanese broadcaster NHK reported that the Osprey was on its way from the Iwakuni US base near Hiroshima in the Yamaguchi region headed for the Kadena base further south in Okinawa.

NHK also cited defence ministry sources as saying that the aircraft was a CV-22 Osprey belonging to the US Yokota air base in Tokyo.

Government spokesman Hirokazu Matsuno said Japan was "aware of information that the US military's Osprey fell out of radar (contact)" near Yakushima Island.

"The government is confirming the extent of damage and will prioritise saving human lives," Matsuno told reporters.

Prime Minister Fumio Kishida suggested his government will not immediately ask the US military to ground its Ospreys or consider doing so for Ospreys used by the Japanese Ground Self-Defense Force, 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.

- Troubled history -

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.

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.

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

- Other crashes -

There have been multiple other crashes of US military aircraft in recent years.

This month five US service members were killed when a helicopter crashed into the Mediterranean during a training exercise.

An F-35 stealth warplane went down in South Carolina in September, with the pilot able to eject.

In April, three US soldiers were killed and another injured when two helicopters returning from a training mission in a remote area of Alaska collided.

The previous month, two US Army helicopters crashed during a nighttime training mission in Kentucky, killing all nine soldiers on board.

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