The Japanese government said Wednesday it had reported to a U.N. Security Council panel a "strongly suspected" case of a transfer of goods between a North Korean tanker and a Commonwealth of Dominica-flagged vessel in the East China Sea in violation of U.N. sanctions.

The tankers were detected by a Japanese Maritime Self-Defense Force P-3C aircraft in the early hours of Saturday. Disclosing photos of the two vessels alongside each other, the Japanese Foreign Ministry said they may have been "doing some kind of work" in dim lighting.

(Defense Ministry)

The tankers were identified as North Korea's "Rye Song Gang No. 1" and Dominica's "Yuk Tung."

The Rye Song Gang is known to have been involved in a ship-to-ship transfer, possibly of oil, in a bid to evade sanctions, according to a U.S. Treasury Department announcement in November. A port entry ban was imposed on the vessel by the U.N. Security Council the following month for its involvement in transporting prohibited goods.

The ministry said in a press release that it believes the ship is now operating under the name "Song Hae" and it "strongly suspects" the vessel is seeking to skirt sanctions.

At the request of the United States, MSDF vessels and aircraft have been patrolling the high seas near Japan since late last year for possible cases of North Korean oil smuggling.

The U.N. Security Council has imposed stepped-up sanctions on North Korea as it continues to test-fire missiles and conduct nuclear tests.