South Korea played down a report Tuesday that the North's leader Kim Jong Un was being treated after surgery, as speculation mounted over his absence from a key anniversary.

Pyongyang marked the birthday of its late founder, Kim's grandfather Kim Il Sung, on April 15 -- by far the most important date in its annual political calendar -- but Kim was not seen in attendance.

Daily NK, an online media outlet run mostly by North Korean defectors, said Kim had undergone a cardiovascular procedure earlier this month and was recovering at a villa in North Phyongan province.

"Excessive smoking, obesity, and fatigue were the direct causes of Kim's urgent cardiovascular treatment," it cited an unidentified source inside the country as saying.

No confirmation of the report was immediately available.

But it triggered widespread speculation, with CNN citing a US official saying that Washington was "monitoring intelligence" that Kim was in "grave danger after a surgery". The report did not specify what the intelligence was.

In a statement, a spokesman for the South's presidential Blue House said: "We have nothing to confirm and no special movement has been detected inside North Korea as of now."

Kim Jong Un's health is frequently the subject of speculation, but little concrete is known about the secretive leader
Kim Jong Un's health is frequently the subject of speculation, but little concrete is known about the secretive leader KCNA VIA KNS / STR

Some South Korean officials raised doubts about the credibility of the sourcing.

Reporting from inside the isolated country is notoriously difficult, especially on anything to do with the North's leadership, which is among its most closely-guarded secrets.

Both the unification ministry -- which handles inter-Korean relations -- and the defence ministry declined to comment.

Moon Chung-in, the security adviser to South Korean President Moon Jae-in, told AFP that he had not heard anything on Kim's health.

The North's state media last reported on Kim's activities on April 12, but previous absences on his part from the public eye have also prompted speculation about his health.

Analysts expressed caution over the Daily NK report.

Ahn Chan-il, a defector-turned-researcher based in Seoul, said heart surgery required sophisticated medical equipment "only accessible in facilities in Pyongyang", adding it was "unreasonable" to transport it elsewhere for the operation.

"Nothing is confirmed at this point, and it's too early to make a conclusion about his condition," he added.

South Korea