Japan on Tuesday said it had detected what appeared to be a Chinese-flagged vessel, 350 km (218 miles) off Shanghai, conducting illegal transfers to a North Korean ship.

“Following a comprehensive assessment, the government of Japan strongly suspects that they conducted ship-to-ship transfers banned by UNSCR,” Japan’s Ministry of Foreign Affairs said in a press release, referring to U.N. Security Council resolutions.

A Japanese P-3 maritime patrol plane detected the vessels, which lay alongside each other connected by hoses, on May 19, with one of the ships flying what seemed like a Chinese flag, it said.

China has repeatedly said it is fully enforcing U.N. sanctions against North Korea and it would punish any Chinese company breaking them.

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