The two vessels were spotted by a Japanese spy plane.
A spokesman for the Chinese foreign ministry revealed Beijing is investigating a Japanese report that a Chinese ship made an unauthorized transfer with a North Korean vessel.
It is the third time this year that Tokyo has reported a cargo transfer by a North Korean vessel in violation of United Nations sanctions over Pyongyang's banned nuclear and ballistic missile programmes.
"Recently, the relevant (Chinese) authorities issued a public notice demanding the strict prohibition of ship-to-ship trafficking in accordance with UN Security Council resolutions", he said.
Punishment of any Chinese entities found to have been involved would be carried out "based on solid evidence and in accordance with Chinese laws and regulations", he said.
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A Japanese P-3C patrol plane saw the North Korean tanker alongside a small ship of unknown registry at a point some 250 kilometers off the coast of Shanghai on February 16, Japan's government said on February 20.
Tokyo informed a UN Security Council sanctions committee that a transfer of goods may have been conducted between the ships, in violation of UN Security Council resolutions. The ministry stressed the importance of the drill, citing November's North Korean ballistic missile activities.
The North Korean UN mission asked Mr Guterres to push the Security Council to welcome, "the process of improved inter-Korean relations and discouraging neighbouring countries from disturbing the process".
The CNN report said, "Speaking in Japan last week ahead of his visit to Pyeongchang for the Winter Olympics, US Vice President Mike Pence said Washington plans to unveil what he called the "toughest and most aggressive" sanctions yet against North Korea".
"Following a comprehensive analysis, the government strongly suspects they were transferring goods, which is banned by United Nations sanctions", said Japan's government, which also released a photo of the ships.