The tilt-rotor CV-22B Osprey crashed on Wednesday off the island of Yakushima with eight on board, in the latest mishap involving the versatile aircraft.

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

It said in a statement that a dive search was conducted after an unidentified object was detected by sonar but that "no clues were found regarding the missing people. We have no other new information on them."

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

"Seven Airmen are in DUSTWUN status meaning 'duty status-whereabouts unknown'. At this time, we can confirm one set of remains has been recovered," US Air Force Special Operations Command said.

Deputy Pentagon Press Secretary Sabrina Singh said in a statement Friday that the unit that suffered the accident is "not conducting flight operations."

It was not clear how many aircraft are affected by the halt, nor how long it will last.

Photos from the area after the incident showed what appeared to be an overturned yellow life raft and other debris including what looked like part of an aircraft rotor blade.

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 off Australia's north coast and in 2000 19 Marines died 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 Osprey flights again -- except for 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 flights 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.

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