A Hong Kong-flagged cargo vessel that recently made a port call in Japan may have breached a ban imposed by Tokyo on the entry of third-country ships that had earlier visited North Korea, investigative sources said Thursday.

According to the sources, a crew member from the vessel "Ocean Skipper" told Japanese police that the ship called at the North Korean port of Rajin in January and February, loaded tens of thousands of tons of coal each time and shipped them to China.

If the statement is true, the vessel violated Japan's unilateral sanctions against Pyongyang, which, among other things, prohibit any ship from entering a Japanese port if it made a previous stop in North Korea.

But Japanese police allowed the Ocean Skipper to depart without initiating necessary procedures for a criminal case. The vessel's captain could have faced up to three years in prison or 3 million yen ($26,600) in fines.

The crew member was questioned by police at Chiba Port, southeast of Tokyo, where the cargo vessel was docked from Nov. 3. The police also found records of the coal shipments from Rajin to China on the vessel, the sources said.

The Ocean Skipper left for Singapore on Monday after loading around 40,000 tons of slag, a substance separated from metal ore during the refining process.

A ship that enters a port in Japan must relay its latest 10 port stops to the Japan Coast Guard. The ship's call at Rajin was not reported by the Ocean Skipper.

Japan imposed an entry ban on third-party ships that called in North Korea after a government decision in February last year. But with more than 50,000 ships entering Japanese ports every year, it is a difficult task for authorities to track the ships' recent port visits.