The Japanese navy is continuing its search for seven crew in the Pacific Ocean after the apparent crash of two helicopters on Saturday night, which killed at least one crewman.
Rescuers have so far recovered the body of one man, as well as black boxes and debris, but are yet to find the others.
Prime Minister Fumio Kishida mourned the tragedy in parliament on Monday.
"It is of utmost regret that we lost our personnel as they engaged in very tough, late-night training," he said.
It's still unclear what exactly caused the crash, which took place near the Izu Islands, which lie not far from the mainland, about 600km (372 miles) south of Tokyo.
Flight recorders retrieved from both aircraft had not revealed technical problems, Japan's Defence Minister Minoru Kihara said.
"Firstly we do our best to save lives," Mr Kihara said on Sunday, adding the helicopters had been "doing drills to counter submarines at night".
The two helicopters were both carrying four crew during the drills on Saturday night.
Communication with one helicopter was lost at 22:38 local time (14:38 BST) off the island of Torishima, broadcaster NHK reported,
One minute later, an emergency signal was received from this aircraft.
After 25 minutes, the military realised communication with the other aircraft was also lost in the same area.
Authorities launched a major search operation using 10 ships and five aircraft to scour the waters.
"We have a 24-hour operation for the search, which took place all day and all night," a navy officer told AFP news agency.
Along with the recovery of one man's body, crews found the flight recorders near each other as well as debris including parts of rotor blades.
The Mitsubishi SH-60K helicopters - based on the Sikorsky Seahawk - mainly operate from naval destroyers.
As there were no other aircraft nor vessels in nearby waters, the navy said involvement of another country in the incident was unlikely.
Japan has boosted defence spending and deepened cooperation with the US, South Korea and other countries in Asia in response to China's growing assertiveness in the region.
In April 2023 a Japanese army helicopter with 10 people on board crashed off Miyako island in southern Okinawa prefecture.
The UH60 helicopter, known as a Black Hawk, had been surveying the local area at the time it disappeared.
A senior Ground Self Defence Force commander, Lt Gen Yuichi Sakamoto, was among those on the flight. There were no survivors.
The US military also maintains a base on Okinawa, a grouping of islands in the East China Sea about halfway between Taiwan and Japan's mainland.
