The U.S. is seeking details about Kim Jong Un’s health after receiving information that the North Korean leader was in critical condition after undergoing cardiovascular surgery last week, a U.S. official said.

The Trump administration wasn’t sure whether Kim was dead or alive, said the official, who asked not to be identified. CNN had earlier reported, citing a U.S. official with direct knowledge of the matter, that Kim may be in "grave danger” after the surgery. The White House declined to comment on the CNN report.

The Daily NK, a Seoul-based website that gathers information from informants inside the isolated nation, separately reported that Kim underwent a "cardiovascular surgical procedure” and was now mostly recovered. It was impossible to immediately verify the report, which the Daily NK said was based on one person inside North Korea.