President Donald Trump says that he appreciates China’s efforts to exert pressure on North Korea, but “it has not worked out.”

Trump tweeted Tuesday, “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!”

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His tweet comes a day after the death of Otto Warmbier, an American student who was returned from North Korea to the U.S. in a coma last week.

Trump said Tuesday that it was a “total disgrace” what happened to Warmbier, but neither doctors nor the administration have offered an explanation for the student’s condition preceding his death.

Trump has called repeatedly on China to help exert pressure on North Korea, particularly with regard to its nuclear ambitions.

President Trump admits that things have “not worked out” with how one country has handled North Korea AP Photo/Wong Maye-E, File