TOKYO, Dec 1 (Reuters) - Japan is concerned that the U.S. military is continuing to fly Osprey aircraft despite its request to ground them until their safety is confirmed after an accident this week, Japan's top government spokesperson said on Friday.

Japan, a key U.S. ally, had sought the suspension of all non-emergency V-22 Osprey flights over its territory after one fell into the sea on Wednesday in western Japan. Japan's Coast Guard has said one person was found and confirmed dead, and the search for the remaining seven aboard continues.

The Pentagon said on Thursday that it was still flying Ospreys for now, and that it was not aware of any official request for their grounding.

Asked about that statement, Japan's chief cabinet secretary, Hirokazu Matsuno, said Tokyo had "officially" made the request.

"We are concerned that despite our repeated requests, and in the absence of sufficient explanation (from the U.S. military), the Osprey continues to fly," he told a news conference.

The Japan Self-Defense Forces (SDF), which also operates Ospreys, has said it would suspend flights of the transport aircraft.

The deployment of the hybrid aircraft in Japan has been controversial, with critics of the U.S. military presence in the southwest islands saying it is prone to accidents.

Japan hosts the biggest overseas concentration of U.S. military power, with the country home to the only forward-deployed American carrier strike group, its Asian airlift hub, fighter squadrons and a U.S. Marine Corps expeditionary force.