"During a combined U.S.-Japanese search and rescue dive in the vicinity of Yakushima, Japan... remains were discovered along with wreckage from the CV-22 mishap," the military said in a statement.

Efforts are underway to recover the remains but "the identities have yet to be determined at this time", the statement said.

"The main priority is bringing the Airmen home and taking care of their family members."

Japan's public broadcaster NHK, citing unidentified sources, said earlier on Monday what appeared to be the front part of the aircraft, possibly including the cockpit, had been found.

It also quoted sources as saying five bodies were found.

The tilt-rotor CV-22B Osprey crashed off the island of Yakushima on Wednesday with eight on board. The body of one crew member was recovered the same day.

Japan's top government spokesman Hirokazu Matsuno declined to comment on the NHK report that more bodies had been found.

"The search and rescue operation is continuing 24 hours a day," Matsuno said.

"At this point, my understanding is that it has not led to the rescue of the remaining seven individuals."

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

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

Three Marines were also killed in 2017 when another Osprey crashed off Australia's north coast and 19 Marines died when their Osprey crashed during drills in Arizona in 2000.

The United States temporarily grounded the aircraft in Japan in 2016 after an Osprey crash-landed off Okinawa, sparking anger among locals.

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

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

NHK quoted a local fisherwoman who said she saw the aircraft crash into the sea, sending up a column of water 100 metres (330 feet) into the air.