Officials said the Lighthouse Winmore had secretly transferred 600 tonnes of refined oil to a North Korean ship.
A UN Security Council resolution bans ship-to-ship transfers of any goods destined for Pyongyang.
The revelations came as China denied claims by President Donald Trump it had allowed oil shipments to the North There is no evidence as, although the Lighthouse Winmore is Hong Kong-flagged, it was leased by a Taiwanese company..
The Chinese government said the accusations against it were “not consistent with the facts”.
“China has never allowed Chinese enterprises nor individuals to violate UN Security Council resolutions imposed on the DPRK,” foreign ministry spokesperson Hua Chunying said.
China handles about 90% of the North’s foreign trade.