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.

The health of North Korea’s leader is one of the state’s most closely guarded secrets, typically only known by a handful of people in the inner circle of leadership. Speculation about Kim had been growing since his unprecendented absence from April 15 celebrations for the birthday of his grandfather and state founder Kim Il Sung, one of the biggest days on the North Korean calendar.

Kim, 36 and a heavy smoker, has been shown in state media in recent months appearing at military drills and riding a white horse on the country’s revered Mount Paektu, where state propaganda says his grandfather used as a guerrilla base to fight Japanese colonial occupiers.

The Unification Ministry said Friday it was “inappropriate” to speculate about the reasons for Kim’s absence. Kim has made 17 public appearances this year that were mentioned in state media — at a pace of a little more than one a week — the ministry said. That’s slightly down from 84 public appearances last year.

North Korea has been battling to prevent a coronavirus outbreak, which has led to cutting back on celebrations and some other major events and for cadres to appear in protective masks in public events with Kim.

The Daily NK is part of a group of not-for-profit agencies affiliated with the Unification Ministry, and has occasionally been contacted by Seoul officials for information. The outlet’s backers include the U.S.’s National Endowment for Democracy, which awarded it $400,000 last year to “raise awareness and understanding of the conditions in North Korea by disseminating accurate, timely and relevant news and information about the country.”

“Given the difficulty of verifying intelligence about North Korea and our mixed track record of predicting exactly what North Korea’s actions signify, we need to avoid jumping to any hasty conclusions just yet,” said Mintaro Oba, a former U.S. diplomat who worked on Korean Peninsula issues.