On Friday, South Korean authorities said they seized a Hong Kong-flagged vessel suspected of transferring oil to a North Korean vessel at sea on October 19.
The vessel, Lighthouse Winmore, transferred refined petroleum products to a North Korean ship in global waters in late October, a foreign ministry official told Reuters.
The vessel, the Lighthouse Winmore, was seized one month after it allegedly ferried oil, South Korean media report.
China denied Friday that it was supplying oil to North Korea, USA Today reported.
Chinese foreign ministry spokeswoman Hua Chunying said, "The recent series of reports on this situation do not conform with the facts".
China rejected the accusations on Friday, stressing that China did not violate United Nations resolutions and had looked into the report of a Chinese ship transferring oil to a North Korean vessel and found it to be inaccurate.
The Lighthouse Winmore, which was chartered by a Taiwanese company, was impounded by South Korean customs authorities at the port of Yeosu on November 24 following an inspection.
UN member countries are banned from making ship-to-ship transfers of prohibited goods for North Korea, according to the UN Security Council (UNSC) Resolution 2375 adopted in September.
China has repeatedly said it is fully enforcing all resolutions against North Korea, despite suspicion in Washington, Seoul and Tokyo that loopholes still exist.
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The U.S. Treasury Department last month sanctioned six North Korean shipping and trading companies and 20 of their vessels, and published photos of what it said was a North Korean vessel on October 19 possibly transferring oil to evade sanctions.
She added: "If there is solid evidence proving that there is on the Chinese side any violation of the Security Council resolutions, China will surely deal with it in accordance with laws and regulations, and not a single case of violation should get away with it".
"China has been completely and strictly implementing Security Council resolutions and fulfilling our worldwide obligations", Hua said at a regular news briefing.
As state-run Xinua media reports, "Hua said that China had immediately investigated the relevant ship and found that it has not docked in any Chinese ports and had no entry and departure records of Chinese ports since August".
Trump has had a mixed strategy with China ever since stepping into the White House earlier this year.
In the New York Times interview, Trump explicitly tied his administration's trade policy with China to its perceived cooperation in resolving the North Korea nuclear crisis. "If they don't help us with North Korea, then I do what I've always said I want to do".
The Hong Kong government said it was "liaising with the Korean parties concerned to obtain further information about the incident, and will take appropriate actions as necessary".
An official of the US State Department said the US government was aware of vessels engaged in such activity involving refined petroleum and coal.
The ban of the four vessels brings the UN's total number of blocked ships to eight.
Beijing, long North Korea's main diplomatic protector, has expressed growing frustration with its nuclear and missile tests.