US President Donald Trump took to Twitter Tuesday to thank China for attempting to resolve tensions with North Korea but declared that Beijing’s “efforts” had failed.

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

The tweet came a day after the death of Otto Warmbier — a US student who returned in a coma from prison in North Korea — and on the eve of a first US-China strategic security dialogue.

Since coming to office, Trump has tried to get Beijing to put pressure on Pyongyang over its nuclear and ballistic missile programs.

It was unclear if the tweet was a signal that those efforts have failed. That conclusion could be seen as a warning of unilateral US action to follow.

While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!

— Donald J. Trump (@realDonaldTrump) 20. Juni 2017

Two B-1B strategic bombers of the U.S. Air Force train over Koreahttps://t.co/K6kqrpczmV pic.twitter.com/PtWhLZLckf

— Defence blog (@Defence_blog) 20. Juni 2017