TOKYO, (AFP) -- The badly decomposed remains of seven people have been found in a suspected North Korean fishing boat that washed up on a Japanese island, a coast guard official said Sunday.

The remains were found on Saturday in a broken vessel on the shore of Sado Island, which lies around 900 kilometres (560 miles) from North Korea across the Sea of Japan.

"Five of the bodies were identified as men but the remaining two could not be identified" as they were badly damaged, the official told AFP.

"There have been similar cases but this was the first discovery of bodies in such a wrecked boat on this island this year," the official added.

Korean alphabet and numbers were painted on the wooden hull, which was broken in half, he said, adding that there was nothing to show their nationality.

Local media said Japanese police and coast guard are investigating the case, suspecting the vessel departed from North Korea.

So far this year, at least 156 suspected North Korean fishing vessels washed up on Japan's coast or were found drifting in Japanese waters, according to the Yomiuri Shimbun.

Experts say some North Korean fishermen are travelling far out to sea in order to satisfy government mandates for bigger catches.

But their old and poorly equipped vessels are prone to mechanical and other problems, including running out of fuel, and there are few ways for them to call for rescue.

Some of the boats wash up on Japanese shores with dead crews, called "ghosts ships" by local media.

Last year, 10 North Koreans rescued from a tiny wooden boat drifting off northern Japan were deported.

In October, around 60 members of a North Korean fishing boat were rescued in the Sea of Japan after it sank following a collision with a Japanese patrol vessel. (AFP)