US President Donald Trump -- who has often hailed China's efforts to put pressure on North Korea -- hit out at Beijing Thursday for failing to cut off Pyongyang's oil supply, saying such moves prevented a "friendly solution" of the crisis.
"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea," Trump said on Twitter.
"There will never be a friendly solution to the North Korea problem if this continues to happen!"
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 that US satellites had spotted Chinese ships selling oil to North Korean vessels dozens of times since October.
It was not immediately clear if 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.
A State Department official later said that the US was aware that "certain vessels have engaged in UN-prohibited activities, including ship-to-ship transfers of refined petroleum and the transport of coal from North Korea."
"We have evidence that some of the vessels engaged in these activities are owned by companies in several countries, including China," the senior official said, adding that oil is vital to the regime and its military, while coal exports have been a primary means of generating revenue.
"We condemn these acts and hope that any UNSC members, including China, work more closely together to shut down smuggling activities," the official added.
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% 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.
The UN Security Council on Thursday meanwhile denied international port access to four North Korean ships suspected of carrying or having transported goods banned by international sanctions targeting Pyongyang, diplomats told AFP.
The ban of the four vessels brings the UN's total number of blocked ships to eight. The United States requested the most recent ban along with measures targeting ships registered in other countries, diplomats said on condition of anonymity.
In recent months, Trump's administration has praised Beijing for its efforts to tame North Korea. China has voted in favour 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.