A Maritime Self-Defense Force ship has spotted a North Korean tanker together with a South Korean cargo ship on the high seas, though the MSDF vessel could not confirm that goods were being transferred in violation of U.N. sanctions, government sources said Sunday.

Tokyo is asking Seoul to investigate whether the South Korean vessel spotted in the East China Sea on May 3 engaged in a transfer with the other vessel, the sources said, adding they had not received an official response from Seoul.

The sources said the MSDF did not observe any noticeable change in the drafts of the two ships, indicating no cargo transfer took place.

Cargo transfers between ships from North Korea and U.N. member states are banned by a Security Council resolution adopted last year in response to Pyongyang’s nuclear and ballistic missile tests.

North Korean ships have been seen in the East China Sea earlier this year apparently transferring goods with tankers registered in Belize and the Commonwealth of Dominica, as well as an unidentified smaller vessel bearing writing linking it to a Chinese port.

There has not been any case so far in which a South Korean ship-to-ship transfer with a North Korean ship was suspected.