BEIJING –  China says it is "highly concerned" about a reported ship-to-ship transfer on the high seas that could violate U.N. sanctions on North Korea.

Foreign ministry spokesman Geng Shuang said Thursday that Beijing is investigating the incident reported by Japan earlier this week.

Geng said China would "seriously deal" with any Chinese individuals or enterprises found to be involved.

Photos of the transfer released by Japan show a North Korean-flagged tanker alongside a smaller ship that appears to be Chinese, with what appear to be hoses running between them.

Japan has reported three such incidents in the past month.

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