A U.S. military helicopter crashed on Japan's southern island of Okinawa on Monday, U.S. Forces in Japan said, an incident which may stoke anger over the concentration of U.S. military bases on the island.
The Japanese defence minister said three of the four people onboard were confirmed to have survived the crash, but the U.S. Air Force said in a news release that the condition of at least four people onboard was unknown.
There were no casualties among local residents, a Japanese official said.
Video footage showed smoke rising from a fire on a remote mountainside.
The air force said an HH-60 helicopter, based in Kadena airbase in Okinawa, crashed in a training area on the U.S. Marines' Camp Hansen and U.S. fire and rescue crews were responding, adding that the helicopter was conducting a training mission at the time.
A private broadcaster earlier said that the aircraft was a CH-46 helicopter.
"This is really regrettable. We are asking the U.S. side for a speedy supply of information," Defence Minister Itsunori Onodera told reporters. "We plan to strongly demand for investigation into the cause of the accident and measures to prevent a recurrence."
Prime Minister Shinzo Abe wants to move the U.S. Marines' Futenma airbase to a less crowded part of the island, but stiff opposition from Okinawa residents is stalling the plan.
Residents of Okinawa, host to the bulk of U.S. military forces in Japan, have long resented bearing what many feel is an unfair share of the burden for the U.S.-Japan military alliance. Many associate the U.S. bases with accidents, crime and pollution.