Two people were critically ill today 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 with the Osumi, a naval transport ship.
All four crew from the fishing boat were pulled from the water.
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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.