Five US Marines missing after two US military planes crashed during refueling operation off the coast of Japan.
Rescue operations were underway, the Japanese defence minister told media.
One crew member has already been rescued and is thought to be in a stable condition, Takeshi Iwaya told reporters.
A spokesman for the Japanese Self-Defense has said another crew member had been found but there were no immediate details of the marine´s condition.
“US military and the Japanese Self-Defense planes and vessels are searching for those still missing… I hope all the members will be rescued safely as soon as possible,” Iwaya added.
The marines were conducting “regularly scheduled training” when the crash occurred around 2 am local time, the 3rd Marine Expeditionary Force said in a statement.
The F/A-18 fighter jet with two crew on board and a KC-130 refueling tanker with five crew — crashed into the sea around 100 kilometres (55 nautical miles) off the cape of Muroto in south-western Japan, Iwaya said.
The crew member rescued had been in the fighter jet, the minister confirmed. Japan´s SDF had deployed nine aircraft and three vessels for the search, he said.
“We are thankful for the Japan Maritime Self-Defense Force´s efforts as they immediately responded in the search and rescue operation,” the Marines said.
A spokesman for the Japanese coastguard said six vessels and an aircraft had been dispatched separately to assist in the rescue efforts.
There are few details about the circumstances of what the Marines described as a “mishap” and an investigation is underway.
During a normal KC-130 refueling operation, the tanker aircraft trails a hose from the back of the plane with a so-called “drogue”, shaped a bit like a windsock, at the end.
The fighter jet then inserts a probe inside the drogue to receive fuel, which it can do at a rate of up to 3,000 pounds per minute, according to a paper prepared for the US Congress.
The US military has about 50,000 troops stationed in Japan and accidents are not uncommon.
In November, a US navy fighter jet crashed into the sea off Japan´s southern island of Okinawa and its two crew members were rescued alive.
And in November 2017, a C-2A “Greyhound” aircraft with 11 people on board went down in the Philippine Sea — eight were rescued and the search was called off for the remaining three after a two-day search.
The US military has also experienced difficulties with its Osprey helicopters, with several emergency landings, a deadly crash and a piece of the chopper falling on the grounds of a Japanese school.