Japan's Foreign Ministry says a Japanese warship has spotted a North Korean tanker and another tanker with a Korean name apparently transferring fuel on the open seas in violation of U.N. sanctions.

The ministry said in a statement Friday that the two vessels are suspected of conducting offshore ship-to-ship transfers banned under U.N. Security Council resolutions. Japan reported a similar sighting earlier this week.

It said Japan has informed the Security Council of the May 24 sighting of North Korean tanker Sam Jong 2 alongside a ship of unknown nationality with a Korean name. A Japanese destroyer spotted the two east of Shanghai.

The U.N. has blacklisted Sam Jong 2.

U.N. sanctions on North Korea limit energy trade and ban offshore transfers of goods to North Korean ships.