This picture taken in April, 2012, shows transport vessel Osumi (L) of Japan's Marine Self-Defense Force in Miyakojima, Okinawa prefecture.
Two people were critically ill Wednesday after their boat collided with a Japanese naval vessel in waters between two of the country's main islands, officials said.
Television footage showed the upturned hull of a small fishing boat in the Seto Inland Sea off Hiroshima after what the coastguard said was a collision at around 8:00 am (2300 GMT Tuesday) with the Osumi, a naval transport ship.
All four crew from the fishing boat were pulled from the water.
A coastguard spokesman said two of them were conscious, but the other two -- including the captain -- were in cardio-respiratory arrest. The term is usually used by first responders of people who have died but have not yet been certified by a doctor.
Defence Minister Itsunori Onodera expressed regret for the incident and pledged his ministry's full cooperation in the coastguard's investigation into its cause.
The Seto Inland Sea is a relatively busy waterway that serves a number of major ports, as well as supporting a vibrant fishery.