China has apparently been secretly selling oil to North Korea in violation of UN sanctions and its own promises to drastically curb trade with the country.

A report from South Korean newspaper Chosun Ilbo on Tuesday said that US spy satellite images show that Chinese ships have transferred oil to North Korean ships in the West Sea around 30 times since October, citing unnamed government sources in the South Korean government. A report from the Financial Times on Wednesday confirmed the story with other diplomatic officials in the region.

In response to the news, President Trump on Thursday tweeted that China had been “caught RED HANDED” selling oil to Pyongyang. “There will never be a friendly solution to the North Korea problem if this continues to happen!” Trump added.

Trump has put China at the center of his North Korea strategy. China is North Korea’s closest trading partner and ally, and as such has enormous leverage over the country’s small and impoverished economy. Trump has argued that China should do more to pressure North Korea to curb its nuclear and ballistic missile programs.

While China has promised to do so and given the green light to harsh sanctions against Pyongyang at the UN in recent months, the latest reports are a reminder that Beijing routinely fails to deliver on its promises when it comes to getting tough with North Korea.

Hua Chunying, a Chinese foreign ministry spokeswoman, said on Wednesday that she was not aware of the Chosun report but denied that Beijing has violated any UN rules.

Experts say that the reports are not conclusive, but are credible and should be taken seriously. Bonnie Glaser, the director of the China Power Project at the Center for Strategic and International Studies, told me that satellite images are “reliable” and that the Chosun Ilbo report is “likely accurate.”

Patrick Cronin, the senior director of the Asia-Pacific Security Program at the bipartisan Center for a New American Security, characterized the reports as “well-informed speculation backed by some compelling evidence.”

“The report is most likely accurate in general but should not be treated as conclusive,” he said.

Trump appears to believe that the evidence is definitive, and that could spell trouble for US-Chinese ties.

If the US believes that China is cheating, sanctions may be coming

Analysts say that the Trump administration could potentially attempt to hold China accountable by rolling out sanctions targeting Chinese businesses and individuals suspected of trading illegally with North Korea — an act that would certainly anger Beijing.

That wouldn’t be unprecedented — Trump sanctioned a Chinese bank, a Chinese company, and two individuals for their illicit financial ties to North Korea in June.

China and the US have diverging interests when it comes to applying pressure on North Korea. China isn’t as worried about North Korea’s nuclear program as it is the potential collapse of the country itself, which would create a refugee crisis on the Korean Peninsula and likely send millions of North Koreans pouring across the Chinese–North Korean border.

Analysts say China also fears that the US would dramatically increase its military presence in the region to deal with the fallout from a North Korean collapse and to secure the country’s nuclear weapons. A US military presence on China’s border is not something China’s leaders want to see. As a result, China helps prop up the North Korean leadership knowing that a stable North Korea serves as a strategic buffer for itself.

The US, on the other hand, is primarily concerned with crippling North Korea’s economy and sapping it of the resources it needs to fund its nuclear and ballistic missile programs.

So far the US has had mixed results in getting China on board with sanctions against North Korea. China has signed on to multiple rounds of harsh sanctions at the UN Security Council in the past few months which, among other things, cap the amount of oil and fuel that countries can send to North Korea.

This latest report, however, suggests that China isn’t necessarily enforcing the rules that it’s signing up for.