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.

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 always fully enforced U.N. resolutions and if there is concrete proof of the resolutions being breached then China will handle it in accordance with the law, its ministry said.

TOKYO (Reuters) - 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 news 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's Foreign Ministry said the information provided was "unclear" and it was hard to reach any definitive conclusions.

China has always fully enforced U.N. resolutions and if there is concrete proof of the resolutions being breached then China will handle it in accordance with the law, the ministry said in a statement sent to Reuters.

Japan's decision to voice its suspicion comes as North Korean leader Kim Jong Un and U.S. President Donald Trump prepared for what may be a breakthrough summit as early as next month.

Japan is urging the United States and other countries to stick to a strict imposition of U.N. sanctions on North Korea until it abandons the development of nuclear weapons and ballistic missiles.

In April, the U.N. Security Council blacklisted dozens of ships and shipping companies over oil and coal smuggling by North Korea, including five based in China. The vessels are subject to a global port ban and must be deregistered.