In compliance with the United Nations sanctions on North Korea, China claimed they had stopped all flow of refined oil to the communist hermit kingdom. However, South Korea has confirmed that they recently captured a Chinese tanker thought to be bringing refined oil products to the North Koreans in secret.

According to USA Today, the Hong Kong “Lighthouse Winmore” was supposedly bound for Taiwan with hundreds of tons of Japanese petroleum, but instead unloaded the oil to a North Korean tanker in international waters in total defiance of UN resolutions:

The ship, chartered by the Taiwanese company Billions Bunker Group, was loaded with 600 tons of refined Japanese petroleum and supposedly bound for Taiwan when it was originally inspected in the South Korean port of Yeosu in October.

However, it is suspected that the ship actually transferred that load to the awaiting North Korean tanker Sam Jong 2 while in international waters on October 19, after leaving port in Yeosu. North Korea is currently under U.N. Security Council sanctions that prohibit it from importing more than 2 million barrels of refined petroleum annually. Ship-to-ship transfers of any goods are also expressly forbidden by U.N. Security Council Resolution 2375, which was passed in September.

“This is one of the main ways in which North Korea uses an illegal network to circumvent U.N. Security Council sanctions,” said a spokesman for the South Korean Foreign Ministry.

The United States submitted a list of ten ships to blacklist to the UN. The Lighthouse Winmore was one of the ten.

Donald Trump tweeted on Thursday about the Lighthouse Winmore being caught, saying China was “caught RED HANDED” and that he was “very disappointed that China is continuing to provide North Korea with refined oil products in the face of their continued agitation of the United States and the UN with their progression of North Korea’s nuclear missile program.

However, Beijing denies that it sent any refined products to North Korea and vowed to punish any Chinese actors found guilty of violating the agreed upon sanctions.

“China has launched an investigation following reports that a Chinese ship transferred oil to a North Korean ship in international waters on October 19,” ministry spokeswoman Hua Chunying said according to South China Morning Post.

“The ship has not stopped at a Chinese port since August, and there is no record of it leaving or entering a Chinese port. We are not aware if the ship has visited ports operated by other nations.”

According to USA Today, South Korea isn’t releasing the crew of the Lighthouse Winmore until their investigation is finished:

Currently, the crew of the Lighthouse Winmore, which consists of 23 Chinese and two Myanmar nationals, remains in South Korean custody. Seoul says the crew will not be released until authorities have completed their investigation. South Korea intends to inform the U.N. Security Council’s sanctions committee of the findings of that investigation.

At this time, it’s hard to say whether or not the Chinese government is illegally assisting North Korea. The relationship between the two countries has been worsening both privately and publicly over the past year, with North Korean dictator Kim Jong Un and Chinese President Xi Jinping insulting one another behind closed doors.

China has also agreed that further sanctions should be passed on North Korea in order to attempt to reign in their antagonism. For Xi, North Korea is a constant weight on China’s international reputation, and continues to throw North Korea into even more dire straights. This includes more sanctions and bans on tourism, which North Korea relies on heavily.

Regardless, many are skeptical of China’s ability to treat North Korea like the problem state that it is. This includes Trump. Should South Korea’s investigation show that China did indeed assist North Korea illegally with oil, then this will create a large rift within the international community, as well as embolden North Korea.