President Donald Trump appears disappointed in China’s efforts to stop continued aggression from North Korea.

“While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out,” he wrote on Twitter on Tuesday afternoon. “At least I know China tried!”

After meeting with President Xi Jinping in April, Trump appeared optimistic that China would help de-escalate North Korea’s actions in the region, even stalling some of his aggressive actions on trade.

“Look, I’ve been talking about China for years,” he told Fox News host Martha MacCallum during an interview. “Now, I speak nicely about China because I really do believe they’re trying to help out with respect to North Korea.”

Trump also addressed reports of North Korea launching missiles in May, asserting that China was “trying hard” to get them to stop.

Trump’s new focus on North Korea might have been sparked by the death of Otto Warmbier — an American student who died, despite coming home to the United States after spending more than a year in a coma in a North Korean prison.

“It’s a total disgrace what happened to Otto,” Trump said on Tuesday. “It should never, ever be allowed to happen. And frankly, if he were brought home sooner, I think the results would have been a lot different.”