North Korean leader Kim Jong Un has not been seen in public since April 11, and now there are reports of a medical crisis. CNN, citing a single unnamed official, said Monday night that the U.S. is monitoring intelligence that Kim, 36, was in “grave danger” after a recent cardiac surgery. Reuters, meanwhile, notes that a website run by North Korean defectors reports Kim underwent the unspecified cardiovascular procedure on April 12 and is recovering at a villa. There was no official confirmation of either report. South Korea’s government on Tuesday morning denied the reports were true, with a spokesman for President Moon Jae-in saying in a statement: “We have nothing to confirm regarding recent media reports about the health problems of Chairman Kim Jong Un of North Korea, and no atypical movement inside North Korea has been detected.” Speculation about Kim’s health began to spike after he missed the celebration of his grandfather’s birthday—and analysts have noted that his sister, Kim Yo Jong, has taken on a higher profile of late.