China will “seriously” deal with any breaches of United Nations Security Council resolutions on North Korea should they be found, a Chinese official said, soon after the third report this month of a ship-to-ship transfer suspected of violating sanctions.

Chinese Foreign Ministry spokesman Geng Shuang said on Saturday that Beijing still “firmly opposes the United States imposing unilateral sanctions and 'long-arm jurisdiction' on Chinese entities or individuals”.

China had said on Thursday it was “highly concerned” about and was investigating the ship-to-ship transfer Japan reported this past week. A Japanese surveillance plane and escort ship saw a North Korean-flagged tanker alongside a smaller ship on February 16 about 250 kilometres (150 miles) off Shanghai in the East China Sea. Photos appeared to show hoses running between the ships. Japan did not identify the nationality of the second ship.

China has agreed to impose increasingly tough sanctions on North Korea over its nuclear weapons programmes, and Geng has said Beijing recently issued a statement explicitly banning ship-to-ship transfers.

Though China is North Korea's biggest trading partner and a traditional friend, ties have soured over the North's nuclear and missile tests and its refusal to return to Chinese-hosted denuclearisation talks.