Japan’s Ministry of Foreign Affairs announced on Tuesday that a Japanese patrol aircraft spotted two ships at sea performing a transfer of goods in violation of U.N. sanctions against North Korea. One of the ships appeared to be flying a Chinese flag.

The incident occurred on the night of May 19 in the East China Sea, about 218 miles southeast of Shanghai.

A Japanese maritime patrol plane photographed a ship identified as the North Korean tanker Ji Song 6 transferring goods with a smaller, as yet unidentified vessel flying what “seemed like” a Chinese flag. The Japanese Foreign Ministry emphasized that precise identification of the second offending vessel has not been made.

The Ji Song 6 is one of the North Korean ships that has been specifically denied port access by the U.N. Security Council. The company that owns it, Phyongchong Shipping & Marine, has been sanctioned by the U.S. Treasury Department.

“Judging from the fact that the two vessels lay alongside each other with their lights turned on at night and connected hoses, both vessels could have been engaged in some type of activity. Following a comprehensive assessment, the Government of Japan strongly suspects that they conducted ship-to-ship transfers banned by United Nations Security Council resolutions,” the Foreign Ministry said.

The Japanese government posted photos of the two ships on Tuesday, as it has done in previous cases of ships caught making suspected illegal transfers at sea. Based on the photos, it appears the two ships were transferring fuel with hoses.