South Korea revealed it seized a Hong Kong-flagged ship last month that it claims secretly transferred oil to a North Korean vessel — a clear violation of international sanctions.

Authorities said Friday that the Lighthouse Winmore had transferred 600 tons of refined petroleum to a North Korean vessel in October in a ship-to-ship transfer in international waters in the East China Sea. They apprehended the boat and its crew when it arrived in the Yeosu port in the southern tip of South Korea on Nov. 24. The boat was officially seized on Dec. 22, according to AP.

"The actions taken will be reported to the UNSC sanctions committee on North Korea in the future," according to the authorities cited by Yonhap, a South Korean news outlet. "This marks a typical case of North Korea shrewdly circumventing UNSC sanctions by using its illegal networks."

U.N. sanctions implemented in September ban ship-to-ship trading with North Korea over the development of its nuclear program, while U.S. Treasury sanctions from November further restrict the kingdom’s shipping and trading networks.

The news of the seized vessel came just after President Trump blamed China for allowing oil into North Korea.

“Oil is going into North Korea,” Trump said in an interview with the New York Times on Thursday, later tweeting that China had been “caught red-handed.”