A HH-60 rescue helicopter crashed in a training area at Camp Hansen with four crew members on board, the U.S. Air Force said in a statement. The status of the crew members is unknown, it said.
The helicopter, which belongs to Okinawa's Kadena Air Base, was on an unspecified training mission, the statement said. It said the cause of the crash was not known.
Television footage showed smoke rising from a spot in the forest, with a mangled object that appeared to be the frame of the helicopter ablaze.
Defense Minister Itsunori Onodera told reporters that he was told that three of the four crew members ejected themselves from the helicopter and survived, but a fourth person was apparently injured and was taken to a hospital. The condition of the fourth crew member was not immediately known.
Onodera said the accident was "regrettable" and that he was asking the U.S. to provide information promptly, conduct a thorough investigation and take preventive measures.
Okinawan prefectural police said there were no reports of injuries or damage outside the base.
The crash comes amid strong local opposition to the U.S. Marine Corps' additional deployment of 12 MV-22 Osprey transport aircraft on the island. About half of the 50,000 U.S. troops in Japan are based on Okinawa under a Japan-U.S. security pact.
Anti-U.S. military sentiment on the island is a longstanding issue, and many residents have complained about base-related crime, noise and accidents.
Local media said the crash revived memories of an accident in 2004, when a CH-53 helicopter from Marine Corps Air Station Futenma crashed into a nearby university building, triggering a huge anti-base uproar although there were no civilian injuries and the crew survived.
A dozen Ospreys were deployed to densely populated Futenma last year, and another dozen just arrived in Japan for planned deployment. The two countries agreed to close Futenma more than a decade ago, but it continues to operate pending the preparation of a replacement site.
"We knew it was going to happen sooner or later," said Kadena Mayor Hiroshi Toyama, referring to Monday's crash. The news came as he and other local officials were meeting to discuss opposition to the new Osprey deployments on Okinawa.
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