Five skeletal bodies and two human heads have been found inside a shipwreck on a Japanese island.

Police found the ship on Sado Island northwest of the Japanese mainland.

Coast Guard official Kei Chinen said the two heads and five bodies were found in the wooden boat’s stern.

Officials believe the boat is from North Korea. The cause of the deaths is still under investigation.

It is not yet clear whether the heads belonged to the five bodies or were from two other people.

Part of a shipwreck found on Sado Island in Japan with several bodies on board, 28-12-2019. Image: Sado Coast Guard Station via AP

The boat, which had letters and numbers written in Korean on the bow, was first spotted by a police officer on Friday afternoon.

Police waited until Saturday before entering it due to unstable weather.

It is the second wooden boat to wash up on the island in recent months.