A US military plane with six people on board crashed into the sea off western Japan on Wednesday, killing at least one crew member and leaving at least two people in the water unaccounted for, AFP reported.

The US military said the incident occurred during a routine training mission near the island of Yakushima.

“The condition of the crew members is currently unknown,” the US Air Force Special Operations Command said in a statement.

Japan’s coast guard said it found what appeared to be the wreckage of a V-22 Osprey vertical takeoff aircraft and a person, later confirmed dead, about 3 km (2 miles) off Yakushima Island.

Fishing boats in the area found three people in the surrounding waters, a representative of the local fishing cooperative said, adding that their condition was unknown.

Another Osprey landed safely at the island’s airport around noon Wednesday afternoon, around the time of the crash, a local government official said.

US forces in the region are still gathering intelligence, the spokesman said.

The United States has about 54,000 troops in Japan, many of them in the strategic southern archipelago, amid growing Chinese military activity in the South China Sea.

The crash happened shortly before 3pm (Japan time) and witnesses said the plane’s left engine was on fire as it approached the airport for an emergency landing, despite good weather and light winds.

The Coast Guard adjusted the number of people on board to six instead of eight as originally announced.

Yakushima is located in Japan’s Kagoshima Prefecture, about 1,040 km southwest of the capital Tokyo, and is known for its fauna and World Heritage-listed forests.

Other Osprey accidents

Japan, which also operates Osprey planes, said Wednesday it had no plans to ground the planes but asked the U.S. military to investigate the crash.

Jointly developed by Boeing and Bell Helicopter, the Osprey can fly both as a helicopter and as a fixed-wing aircraft and is used by the US Navy and the Japan Self-Defense Force.

The Osprey’s move to Japan was controversial, with critics claiming it was prone to accidents. The US and Japanese militaries say it’s safe.

In August, a US Osprey crashed off Australia’s northern coast while transporting troops during a routine military exercise, killing three US sailors.

Another plane crashed into the ocean off Japan’s southern island of Okinawa in December 2016, leading to a temporary suspension of military flights with that plane.