BEIJING (AP) — China will "seriously" deal with any breaches of U.N. 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.

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

China had said 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 Feb. 16 about 250 kilometers (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 program, 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 denuclearization talks.