The leisure fishing boat with four aboard and the naval vessel Osumi belonging to the Maritime Self-Defense Force collided in the Seto Inland Sea off Hiroshima prefecture, the Japanese Defense Ministry and the coast guard said.
The four on the boat including its captain were thrown into the waters after the collision but the Osumi crew managed to rescue them, Kyodo News reported. The captain and another person were in critical condition while the two were not hurt, the report said,
The cause of the collision was not immediately known. Authorities told Kyodo the sea was calm and the weather was fair at the time of the morning incident.
"As the supervisor (of the MSDF), I find it highly regrettable that such an accident occurred. We will cooperate fully with the Coast Guard investigation," Japanese Defense Minister Itsunori Onodera told reporters.
The 8,900-ton Osumi was on its way to a shipyard in Okayama prefecture for a regular inspection when the collision occurred. It had 135 crew members aboard and is equipped with medical facilities.
Kyodo said the Osumi was sent to the Philippines last November for rescue work during the devastating caused by a typhoon.