Seoul: South Korea downplayed 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 Pyongan 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.

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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 spokesperson 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.”

However, South Korea downplayed reports stating that North Korean leader Kim Jong Un was not gravely ill, two South Korean government sources said on Tuesday, following reports he had undergone a cardiovascular procedure and was now in “grave danger”.

Kim was receiving treatment after undergoing the procedure, a South Korean media report said late on Monday, amid speculation over Kim’s health following his absence from a key anniversary event this month.

Daily NK, a Seoul-based speciality website, cited unidentified sources inside the isolated state saying Kim is recovering at a villa in the Mount Kumgang resort county of Hyangsan on the east coast after undergoing the cardiovascular procedure at a hospital on April 12.