The revelation comes days after South Korea said it was holding a Hong Kong-flagged ship and crew members over transferring petroleum products to a North Korean vessel in global waters in October.
The Seoul government announced Friday it is investigating a Hong Kong-flagged ship named the Lighthouse Winmore for allegedly transferring oil to a North Korean vessel in worldwide waters on October 19.
OFAC said it had found three Chinese companies had cumulatively exported about $650m (about £483m) worth of goods to North Korea and cumulatively imported more than $100m (about £744m) worth of goods between January 2013 and the end of August 2017.
The ship, loaded with the Japanese refined petroleum, was claimed to be heading to Taiwan.
South Korean authorities seized a second oil tanker for allegedly supplying fuels to North Korea at sea, the South's media reported on Sunday.
"The government has been in close consultations with related countries and ministries to thoroughly implement the sanctions by the UN Security Council", the spokesman said.
The Sam Jong 2 was one of four ships barred from worldwide ports by the UN Security Council over suspicions that it had transported goods banned under the sanctions measures.
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The statement did not address whether or not Russian ships had transferred the fuel.
Under United Nations Security Council resolution 2375, which was adopted in September, companies or organizations of member states are not permitted to carry out the ship-to-ship transfer of any goods that are bound for North Korea.
Both China and Russian Federation are the DPRK's long-standing energy patrons, as North Korea has no domestic oil and gas production of its own.
"This marks a typical case of North Korea shrewdly circumventing UN Security Council sanctions by using its illegal networks", he told journalists.
Her remarks came after U.S. President Donald Trump, in a Twitter post Thursday, wrote, "Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea".
Three sanctions have targeted North Korea this year, taking aim at iron, coal, fishing, textiles, oil and refined petroleum products.