Amid a report that U.S. officials were monitoring intelligence that North Korean leader Kim Jong Un may have fallen seriously ill after a surgical procedure, South Korea says it could not corroborate the information.

CNN, citing unnamed U.S. officials, reported Monday that Kim had undergone a cardiovascular surgery earlier this month and intelligence suggested that the leader may be in "grave danger."

The North Korean leader, believed to be 36-years-old, is a heavy smoker and obese.

On April 15, Kim was not present at a state celebration to mark the birthday of his late grandfather, Kim Il Sung, according to CNN.

Kang Min-seok, a spokesman for the Blue House, the office and residence of South Korea President Moon Jae-in, however, said officials there have not detected unusual activity or signs coming from Pyongyang about Kim's health.

South Korea:No reason to think Kim Jong Un gravely ill despite U.S. media report

Undated photos from the North Korean government provided as late as April 12 appear to show Kim inspecting an air defense unit. Other photos provided April 11 appear to show Kim at a politburo meeting.

The CNN report cited Daily NK, a Seoul-based website run by North Korean defectors, which reported that Kim's surgery took place April 12. Daily NK's story is based on a single source inside North Korea.

Here's a look at the most recent known photos of Kim and the April 15 events in North Korea.

Contributing: Kim Hjelmgaard in London

Follow USA TODAY's Ryan Miller on Twitter @RyanW_Miller