US President Donald Trump - who has often hailed China's efforts to put pressure on North Korea - hit out at Beijing on Thursday for failing to cut off Pyongyang's oil supply.
He said such moves prevented a "friendly solution" of the crisis.
"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Mr Trump said on Twitter.
"There will never be a friendly solution to the North Korea problem if this continues to happen!"
Mr Trump did not directly threaten to launch military action to resolve the crisis, but in recent months, Washington has promised to "utterly destroy" the regime of Kim Jong-Un if war breaks out.
South Korean newspaper Chosun Ilbo, citing government sources in Seoul, reported earlier this week US satellites had spotted Chinese ships selling oil to North Korean vessels dozens of times since October.
It was not immediately clear if Mr Trump was referring to the report or US intelligence in his tweet, or if he was accusing China - the North's main ally - of directly violating sanctions targeting Pyongyang.
Last week, the United Nations Security Council - with China's backing - slapped new sanctions on North Korea that will restrict oil supplies vital for Pyongyang's missile and nuclear programs.
The US-drafted resolution bans the supply of nearly 75 per cent of refined oil products to North Korea, puts a cap on crude deliveries and orders North Korean nationals working abroad to be sent back by the end of 2019.
China is Pyongyang's main oil supplier.
In recent months, Mr Trump's administration has praised Beijing for its efforts to tame North Korea. China has voted in favor of three UN Security Council resolutions strengthening sanctions on the North since the summer.
Washington remains convinced that only pressure from the government of Chinese President Xi Jinping will persuade Kim to back down and negotiate an end to the nuclear standoff.
Beijing "has applied certain import bans and sanctions, but it could and should do more", US Secretary of State Rex Tillerson said Wednesday in a column published in The New York Times.
