Japan has reported to the United Nations a suspected ship-to-ship goods transfer incident involving a North Korean tanker in the East China Sea in violation of U.N. sanctions, the Defense Ministry said Wednesday.

The North Korean-flagged tanker Yu Phyong 5 was initially spotted beside a small vessel of unknown nationality on June 21 in waters around 400 kilometers off Shanghai.

The following day, the North Korean tanker was seen beside a similar vessel around 450 kilometers off Shanghai. The vessel, carrying a flag similar to that of China, may have been the same as the one seen the previous day, according to the ministry.

The Maritime Self-Defense Force spotted the vessels.

In each case, a hose was connected between the North Korean tanker and the other vessel, leading the government to “strongly suspect” that they were engaged in ship-to-ship transfers banned by the United Nations.

The Yu Phyong 5, subject to an asset freeze, is prohibited from entering any foreign port under a U.N. resolution.

Japan has reported a series of suspected cases of ship-to-ship transfers in recent months.