South Korean customs authorities inspected the vessel, Lighthouse Winmore from Hong Kong, when it entered country's port Yeosu on November 24 after allegedly transferring 600 tonnes of refined oil to a North Korean vessel on October 19, the Yonhap news agency reported.
On Wednesday, The Financial Times newspaper reported citing diplomats that Chinese vessels were secretly trading oil products with North Korea in violation of UN Security Council (UNSC) resolution 2375 passed in September.
In November, however, satellite photos provided by US Treasury showed Chinese and North Korean ships linked to each other at sea and allegedly trading oil.
Tensions on the Korean Peninsula have escalated this year as North Korea continues to conduct nuclear and ballistic missile tests in violation of UNSC resolutions. Several rounds of sanctions have already been imposed on the country.