Donald Trump has publicly accused China of supplying oil to the North Korean regime, claiming it has been "caught red-handed".
According to a report from Fox News, drawing on information from the South Korean newspaper Chosun Ilbo, US spy satellites have recently captured photos of Chinese ships illegally selling oil to North Korean boats. This has happened 30 times since October.
Satellite images released by America's Department of Treasury appear to show vessels from the two countries trading oil in the West Sea.
Such activity would be illegal, as the United Nations Security Council has banned North Korea from importing natural gas and the country's crude oil imports have been capped.
Minutes after Fox News aired its report on television today, Mr Trump posted a tweet dressing down China and saying the possibility of a "friendly solution" to North Korea's nuclear ambitions had diminished.
"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," he said.
Mr Trump has recently praised China for helping in efforts to keep Kim Jong-un under control. The United States is convinced that only pressure from China's government will convince the dictator to back down.
Earlier, China denied the report, saying there had been no oil sales violating UN sanctions.
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 co-operation of China, North Korea's neighbour 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.
Last week the UN Security Council unanimously imposed new sanctions on North Korea for a recent intercontinental ballistic missile test, seeking to further limit its access to refined petroleum products and crude oil.
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.
Fox News reports the satellite images in question identify the ships. One of the North Korean vessels, the Rye Song Gang 1, is seen connected to a Chinese boat, and was already suspected of transferring oil to dodge sanctions.
China is the main source of North Korea's fuel, though it stopped giving the regime iron ore, coal, lead and oil during October and November.
Robert Kelly, a professor at Pusan National University in South Korea, told the UK Telegraph China was capable of surreptitiously sending oil to Kim Jong-un.
"There is a lot of under-the-radar on the Chinese side. Beijing does not police the border strictly or enforce the sanctions toughly," he said.
TRUMP MOCKS VANITY FAIR
The developing international crisis didn't stop Mr Trump from taking time out of his presumably busy schedule to blast one of his favourite media targets.
In another tweet, the president mocked Vanity Fair for apologising to his opponent from the 2016 election, Hillary Clinton, over a satirical video that suggested ways for her to spend her now ample free time.
Mr Trump said the magazine was "bending over backwards". He also claimed Anna Wintour, a supporter of Ms Clinton during the election, was "beside herself in grief" and "begging for forgiveness".
Ms Wintour is actually the editor-in-chief of Vogue, not Vanity Fair, though she is also the artistic director at Conde Nast, which publishes both magazines.
copped some flak over the satirical video called "Six New Year's Resolutions for Hillary Clinton", in which it suggested she work on a sequel to her book, take photos in the woods or take up a new hobby, such as knitting. "Literally anything that will keep you from running again," it said.
The clip provoked a furious backlash, most notably from actress Patricia Arquette, who tweeted in all-caps: "STOP TELLING WOMEN WHAT THE F**K THEY SHOULD DO OR CAN DO. Get over your mummy issues."
"It was an attempt at humour and we regret that it missed the mark," Vanity Fair responded. Little did it know the president himself would weigh in.