Vessels involved in North Korea's export of coal, which is banned under U.N. resolutions, have made repeated port calls in Japan, possibly as a way of evading the embargo, data obtained by Kyodo News showed Sunday.

Such vessels have made over 100 port calls in Japan since the U.N. Security Council banned North Korean coal exports in August 2017 following a series of ballistic missile tests in violation of earlier resolutions, records from private firms that track shipping and the Japan Coast Guard showed.

The vessels, many of which are Panamanian-flagged, visited Russia and China before or after they came to ports in northern and northeastern Japan, in a possible attempt to disguise the origin of their cargoes.

South Korea began cracking down on Pyongyang's coal shipments to the country in August 2018, prohibiting 10 vessels used to carry coal — the reclusive state's biggest foreign currency earner — from entering its ports. Since then, six of them have come to Japan, with one changing its name and registration.

Port inspections of the vessels did not find any banned materials, according to the coast guard, though experts say such ships often appear to be engaged in legitimate operations and smuggled cargoes are not easy to find.

Japan's special law for regulating Pyongyang's shipping only applies to North Korean-flagged vessels.