President Donald Trump lashed out at China on Thursday, accusing the country of undermining economic sanctions meant to pressure North Korea into giving up its nuclear weapons program after a report claimed Chinese ships were spotted transferring oil to tankers under the Korean flag.

"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Trump tweeted from South Florida, where he is spending his holiday vacation. "There will never be a friendly solution to the North Korea problem if this continues to happen!"

The tweet came following a report on Tuesday in the South Korean publication The Chosun Ilbo purporting to show satellite imagery of Chinese vessels transferring oil to North Korean ships at sea in violation of international sanctions. Fox News reported on the story on Wednesday.

China dismissed the Chosun Ilbo report and denied violating the sanctions.

"The situation you have mentioned absolutely does not exist," Chinese Defense Ministry spokesman Ren Guoqiang told reporters, according to Reuters. Chinese customs also said this week that it exported no oil products – including fuel oil, jet fuel, diesel or gasoline – iron ore, coal or lead to North Korea in November.

The U.S. has been pressuring China to enforce an oil embargo against the North in the hopes of reducing tensions on the peninsula stemming from Kim Jong Un's increasingly provocative weapons testing. U.S. analysts say North Korea achieved several critical steps toward its goal of becoming a nuclear power this year, successfully launching an intercontinental missile and miniaturizing a nuclear warhead so it could be mounted to such a missile.