Two Japanese navy helicopters carrying eight crew members crashed in the Pacific Ocean south of Tokyo during a nighttime training flight after possibly colliding with each other, the country's defence minister said on Sunday.

One crew member who had been recovered from the waters was later pronounced dead, while rescuers searched for seven others who were still missing.

The two SH-60K choppers from the Maritime Self Defence Force were carrying four crew each and lost contact late Saturday near Torishima island about 600 kilometers (370 miles) south of Tokyo, Defence Minister Minoru Kihara told reporters.

The cause of the crash was not immediately known, but the two helicopters might have collided with each other before crashing into the water, Kihara said.

Rescuers have recovered a flight data recorder, a blade from each helicopter, and fragments believed to be from both choppers in the same area, signs that the two SH-60Ks were flying close to each other, Kihara said. Officials will analyse the flight data to try to determine what led to the crash.

The MSDF deployed eight warships and five aircraft for the search and rescue of the missing crew.

The helicopters, twin-engine, multi-mission aircraft developed by Sikorsky and known as Seahawks, were modified and produced in Japan by Mitsubishi Heavy Industries. They were on nighttime anti-submarine training in the waters, Kihara said. One lost contact at 10:38 p.m. (1338 GMT) and sent an automatic emergency signal a minute later.

Only one distress call was heard — another sign the two helicopters were near the same place, because their signals use the same frequency and could not be differentiated, Kihara said.

One belonged to an air base in Nagasaki, and the other to a base in Tokushima prefecture.

The SH-60K aircraft is usually deployed on destroyers for anti-submarine warfare, but is also used for search and rescue and other missions. Japan has about 70 of the modified helicopters that are license-produced by MHI.

Saturday's training only involved the Japanese navy and was not part of a multinational exercise, defence officials said. They said no foreign aircraft or warships were spotted in the area.

Japan, under its 2022 security strategy, has been accelerating its military buildup and fortifying deterrence in the southwestern Japanese islands in the Pacific and East China Sea to counter threats from China's increasingly assertive military activities. Japan in recent years has conducted its own extensive naval exercises as well as joint drills with its ally the United States and other partners.

Saturday's crash comes a year after a Ground Self-Defence Force UH-60 Blackhawk crashed off the southwestern Japanese island of Miyako, due to an engine output problem known as “rollback”, leaving all 10 crew members dead, which shocked the nation. In January 2022, a Air Self-Defence F-15 fighter jet crashed off the northcentral coast of Japan, killing two crew.

In 2017, a Japanese navy SH-60J, an earlier generation Seahawk, crashed during a nighttime flight training off Aomori, due to human error. In the US, a fatal crash of a MH-60S Seahawk during training off California in 2021 was attributed to mechanical failure from unsuspected damage during maintenance, according to the Navy.

Japan's NHK public television said no weather advisories were issued in the area at the time of Saturday's crash.