Japan has said a military patrol plane and an escort vessel observed the alleged transfer in the East China Sea last week.
It is the third time this year that Tokyo has reported a cargo transfer by a North Korean vessel in violation of UN sanctions over Pyongyang's banned nuclear and ballistic missile programmes.
"China attaches high importance to the situation and is currently investigating this," Chinese foreign ministry spokesman Geng Shuang told a regular press briefing.
He added that China would deal with any individuals or businesses suspected of violating the resolutions "in strict accordance with China's laws and regulations, based on all evidence".
Japan said late Tuesday its navy found the Yu Jong 2, a North Korean-flagged tanker, alongside a small vessel of "unknown nationality" around 250 kilometres (155 miles) offshore from Shanghai.
Four Chinese characters reading "Min Ning De You 078" -- meaning "Fujian Province, Ningde City, oil tanker 078" in English -- were on the small vessel, the Japanese foreign ministry said.
Tokyo said it had reported the incident to the Security Council and shared information with relevant countries.
The incident comes as global attention is focused on North Korea's participation in the ongoing Winter Olympics hosted by the South, in a diplomatic push by Kim Jong Un's isolated regime.
Pyongyang is subject to a series of UN Security Council sanctions, including one prohibiting all member states from facilitating or engaging in ship-to-ship transfers of goods to or from North Korean-flagged vessels.
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