United States President Donald Trump on Friday said he was disappointed with China for allowing oil to be supplied to isolated North Korea despite the restrictions imposed on Pyongyang. China was caught “red-handed”, Trump said on Twitter.

“There will never be a friendly solution to the North Korea problem if this continues to happen,” Trump said on Twitter.

“I have been soft on China because the only thing more important to me than trade is war,” Trump told The New York Times when he was asked to explain his tweet. “If they are helping me with North Korea, I can look at trade a little bit differently, at least for a period of time. And that is what I have been doing. But when oil is going in, I am not happy about that.”

China on Thursday refuted reports about its ships violating restrictions imposed by the United Nations on supplying oil to North Korea. A South Korean newspaper had quoted officials who claimed that Chinese and North Korean vessels were collaborating to transport oil to Pyongyang.

On September 12, the United Nations Security Council banned textile exports and imposed curbs on shipments of oil products after North Korea’s latest nuclear test. North Korea was condemned globally for conducting its sixth nuclear test since 2006 on September 3. The country claimed it had tested an advanced hydrogen bomb.