South Korea briefly seized and inspected a Hong Kong-registered ship it suspected of transferring oil products to North Korea, an official has said.
China denied the allegations, saying reports that it sold oil to North Korea did "not accord with the facts".
The claims come days after Donald Trump criticised China over claims that three companies had exported and imported millions of dollars of goods and oil to North Korea.
Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea. There will never be a friendly solution to the North Korea problem if this continues to happen!
— Donald J. Trump (@realDonaldTrump) December 28, 2017
The South Korean official said on Friday that the Lightouse Winmore, chartered by a Taiwanese company and carrying oil products from South Korea, had transferred part of its cargo to a North Korean ship on 19 October.
The ship had headed towards its purported destination in Taiwan, but on reaching international waters transferred some of its 600 tonne cargo to the Sam Jong 2 and three other North Korean vessels, the South claimed.
It was seized and inspected by customs authorities when it returned to South Korea's Yeosu port, the official said.
"This marks a typical case of North Korea shrewdly circumventing UN Security Council sanctions by using its illegal networks", he told journalists.
"The actions taken will be reported to the UN Security Council sanctions committee on North Korea in the future."
Three sanctions have targeted North Korea this year, taking aim at iron, coal, fishing, textiles, oil and refined petroleum products.
The Sam Jong 2 was one of four ships barred from international ports by the UN Security Council over suspicions that it had transported goods banned under the sanctions measures.
But images from the US Treasury Department's Office of Foreign Assets Control (OFAC), released at the end of November, appear to show an attempt between North Korea and China to conduct a ship-to-ship transfer, possibly of oil, to evade sanctions.
OFAC said it had found three Chinese companies had cumulatively exported about $650m worth of goods to North Korea between January 2013 and the end of August 2017.