The area where the wreck happened is known for heavy maritime traffic, the Japanese coast guard said. About 400 to 500 ships pass through the zone each day, the service said.
The last known fatal incident there was in September 2015, when a South Korean vessel and a Japanese container ship collided, Japan's coast guard reported. Six Japanese crew were declared dead.
In such a busy shipping lane, the ships may have been in a "restricted navigation" situation, meaning they would have had o observe strict rules for movement and positioning in relation to other vessels in their vicinity, said Carl Schuster, former director of operations at the US Pacific Command's Joint Intelligence Center and a Hawaii Pacific University professor.
Those rules leave vessels little room to maneuver, as turning away from one ship could place a vessel at risk of an even more serious incident with another, Schuster said.
The area has seen two other major collisions in the past five years.