Seoul has dismissed a query by Japan over whether a South Korea-flagged vessel may have been involved in the transfer of goods to North Korea on the high seas, in violation of U.N. sanctions, according to the Japanese government.

Tokyo asked Seoul to look into the matter after a Japanese Maritime Self-Defense Force ship on surveillance duty spotted a North Korean tanker alongside a South Korean-registered cargo vessel in the East China Sea on May 3.

"It is not normal to see a ship come alongside another ship on the high seas. But South Korea said it has not confirmed any ship-to-ship transfer of goods. I think that is the case," Defense Minister Itsunori Onodera told reporters Tuesday.