Japan’s Foreign Ministry said Tuesday that a Japanese navy surveillance aircraft has spotted a suspected Chinese-flagged ship apparently transferring fuel to a North Korean tanker on the open seas.

A ministry statement said that the two vessels were suspected of engaging in offshore ship-to-ship transfers banned under U.N. Security Council resolutions.

It said Japan has informed the U.N. Security Council of the May 19 sighting of the North Korean-flagged tanker, Ji Song 6, moored alongside the ship carrying a flag believed to be Chinese in open seas in the East China Sea.

Four photos taken by the Maritime Self-Defense Force were released, including two showing the ships alongside each other, connected by a hose.

The U.N. has blacklisted the North Korean tanker.

The disclosure of the alleged offshore transfer comes as preparations for an expected summit between President Donald Trump and North Korean leader Kim Jong Un appeared to be in their final stages.

U.N. sanctions on the North limit energy trade and ban offshore transfers of goods to North Korean ships as part of international efforts to pressure Pyongyang into giving up its nuclear weapons.

Japan and the U.S. have said the sanctions must be enforced strictly to be effective, and asked China in particular to tighten enforcement.

(AP)