"Caught RED HANDED - very disappointed that China is allowing oil to go into North Korea. There will never be a friendly solution to the North Korea problem if this continues to happen!" Trump wrote in a post on Twitter.
China earlier on Thursday said there had been no UN sanction-breaking oil sales by Chinese ships to North Korea after a South Korean newspaper said Chinese and North Korean vessels had been illicitly linking up at sea to get oil to North Korea.
The Trump administration has led a drive to step up global sanctions on North Korea in response to Pyongyang's efforts to develop nuclear-tipped missiles capable of hitting the United States.
Washington says the full cooperation of China, North Korea's neighbor and main trading partner, is vital to the success of this effort, while warning that all options are on the table, including military ones, in dealing with North Korea.
The UN Security Council last week unanimously imposed new sanctions on North Korea for a recent intercontinental ballistic missile (ICBM) test, seeking to further limit its access to refined petroleum products and crude oil.
The UN resolution seeks to ban nearly 90 percent of refined petroleum exports to North Korea by capping them at 500,000 barrels a year.
The US-drafted resolution also caps crude oil supplies to North Korea at 4 million barrels a year and commits the Council to further reductions if North Korea were to conduct another nuclear test or launch another ICBM.
Documents seen by Reuters this month showed Washington called on the Security Council to blacklist 10 ships for circumventing sanctions by conducting ship-to-ship transfers of refined petroleum products to North Korean vessels or transporting North Korean coal.
China and Russia subsequently asked for more time to consider the proposal.
In September, the Security Council put a cap of 2 million barrels a year on refined petroleum products exports to North Korea.
China has repeatedly said it is fully enforcing all resolutions against North Korea, despite suspicion in Washington, Seoul and Tokyo that loopholes still exist.
Asked at a regular briefing whether Chinese ships were illegally providing oil to North Korean ships, Chinese Defence Ministry spokesman Ren Guoqiang reiterated that China, including the military, strictly enforced UN resolutions.
"The situation you have mentioned absolutely does not exist," he said.
South Korea's Chosun Ilbo newspaper this week quoted South Korean government sources as saying that US spy satellites had detected Chinese ships transferring oil to North Korean vessels around 30 times since October.
US officials have not confirmed details of the report, but a US State Department official suggested on Wednesday that such transfers could still be going on.
“Ship-to-ship transfers ... remain a concern as part of North Korea’s sanctions evasions activities,” the official said, while declining to provide details.
A State Department spokesman, Michael Cavey, reiterated on Wednesday that the United States had called on all countries to cut economic ties with North Korea.
"We urge China to end all economic ties with the DPRK, including tourism, and the provision of any oil or petroleum products, and expel all DPRK workers," he said, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.
Harry Kazianis, director of defence studies at the conservative Center for the National Interest, said China would "never, ever, enforce the sanctions to the satisfaction of President Trump," in spite of the effort the US president had invested in developing a personal relationship with China's Xi Jinping.
"With President Trump’s latest Tweet it seems the ‘Bromance’ between him and President Xi is finally over," he said.
"This was always bound to happen. China is actually more afraid of North Korea than America," Kazianis said, citing Chinese concerns about instability or collapse in North Korea if sanctions were fully applied.
US Democratic Senator Ed Markey, a member of the Senate Foreign Relations Committee, said on Twitter the North Korean threat had only increased since Trump took office and he had to find a way to get China to cut off crude oil supplies.
"The solution is a coherent strategy, not bluster," he said.
