The tilt-rotor CV-22B Osprey went down Wednesday off the island of Yakushima on a routine training mission with eight crew on board.

One man was found and later declared dead the same day, but the coastguard said Friday that the other seven remained unaccounted for despite a massive search.

"To date, there are no new leads to the missing individuals," it said in a statement.

"Today, in addition to the search by patrol vessels and aircraft, we plan to continue dive searches including areas where the side-scan sonar survey yesterday showed echo images that are different from those on the seafloor."

On Thursday, divers investigated other unidentified objects found by sonar in waters around 30 metres (100 feet) deep that turned out to be rocks.

Photos from the area after the incident showed what appeared to be an overturned yellow life raft and other debris including what was thought to possibly be part of a propeller.

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

Broadcaster NHK quoted a local fisherwoman as saying she saw the aircraft crash into the sea, sending up a column of water as high as 100 metres.

The Osprey, which can operate like a helicopter or a fixed-wing turboprop plane, has suffered a string of fatal accidents.

In August, a crash in northern Australia killed three US marines while four more died in another crash in Norway last year during NATO training exercises.

Three Marines died in 2017 when another 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.

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

On Thursday, Defence Minister Minoru Kihara said he had asked the US military to suspend flights again and that Japan's military had halted using its own Ospreys pending safety checks.

The US military, which has around 54,000 personnel in Japan, has yet to comment on the suspension request or on the cause of the crash.