Japan - Rescuers scoured waters off Japan on Thursday for seven missing US Air Force personnel whose Osprey crashed during a training exercise in the latest incident involving the tilt-rotor military aircraft.

Japan's defense minister said he had requested that US forces in the country suspend Osprey flights in the wake of the deadly incident.

One unconscious person was found in the sea and later declared dead after the aircraft crashed off the island of Yakushima on Wednesday, according to the Japanese coastguard.

Air Force Special Operations Command said eight crew had been aboard the CV-22B Osprey in the "routine training mission" out of Yokota Air Base in Japan.

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"The cause of the mishap is currently unknown," it said in a statement Wednesday, with emergency personnel "on scene conducting search and rescue operations."

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."

Photos released by the coastguard showed what appeared to be an overturned yellow life raft and other debris in the water off Yakushima, which lies south of Japan's southernmost main island of Kyushu.

A Japanese coastguard spokesman told AFP on Thursday that the search operation had continued through the night and involved six patrol ships and two aircraft. Police and local rescuers were also involved, and the coastguard said it would use special sonar devices to scan the sea floor.

The coastguard had initially said eight crew were on board before revising the number down to six and then back to eight.

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.

On Thursday, Defence Minister Minoru Kihara said he had asked the US military to suspend flights again following the latest crash.

"After receiving the first news, we searched to save lives, and this morning, we made the request to the USFJ's commander," Kihara told an upper house diplomacy and defense committee, referring to United States Forces Japan.

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Japan requested that US forces cease Osprey flights until their "safety is confirmed, except for search and rescue operations," Kihara said. "We are requesting swift disclosure of information about situations surrounding the accident."

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

Government spokesman Hirokazu Matsuno said Japan's military had already suspended flights of its own Ospreys "until safety is confirmed" while expressing condolences over the crash.

"I saw the Osprey fly toward the Yakushima airport and then start rotating once or twice. Soon after, orange light radiated off of it, and barely 10 seconds passed before it fell into the ocean. A column of water, maybe 50 or 100 meters high, splashed up," fisherwoman Kayo Ito told broadcaster NHK.

"I can only imagine how much bigger of a disaster it might have been had the Osprey crashed into a ship or nearer the island. [The incident] worries me," she said.