Tokyo's defence minister has called on the US military in Japan to ground its Ospreys pending safety checks, but the Pentagon said it was still operating the aircraft and had not received a suspension request.

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.

- Other crashes -

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 United States to temporarily ground the aircraft in Japan after the accident sparked anger among locals.

- Still operating -

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

But Pentagon spokeswoman Sabrina Singh told a press briefing in Washington on Thursday that she was not aware of any official request and that "the Ospreys are still operating in Japan", according to a transcript.

She added that "if the investigation concludes that there need to be additional steps taken, we'll... certainly do that, but at this time, the investigation is underway on what happened."

Kihara told reporters on Friday he had "concerns about continuation of Osprey's flight despite not enough explanation over flight safety".

Singh said Washington was "eternally grateful for the government of Japan and their Coast Guard for helping in our search and recovery efforts".

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