South Korea played down a report on 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". That 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."And the South's Yonhap news agency cited an unnamed government official saying that reports Kim was seriously ill were "not true".Previous absences from the public eye on Kim's part have prompted speculation about his health.In 2014 he dropped out of sight for nearly six weeks before reappearing with a cane. Days later, the South's spy agency said he had undergone surgery to remove a cyst from his ankle."No-one knows what's going on inside North Korea," said Martyn Williams, who is affiliated with the 38 North research website."Kim Jong Il had been dead several days before it was announced and it took everyone by surprise," he tweeted. "Kim Jong Un has been 'missing' before, and has always reappeared. That said, his absence this week was more notable."No-one knows what's going on inside North Korea. Kim Jong Il had been dead several days before it was announced and it took everyone by surprise. Kim Jong Un has been "missing" before, and has always reappeared. That said, his absence this week was more notable.— Martyn Williams (@martyn_williams) April 21, 2020 Closely guardedThe North Korean leader has not made a public appearance since presiding over a meeting of the Workers' Party politburo on 11 April.His powerful sister Kim Yo Jong was named as an alternate politburo member and the meeting called for stronger measures against the coronavirus pandemic.Pyongyang has closed its borders and imposed tight restrictions to protect itself from the virus that emerged in neighbouring China and has since swept the world, and insists it has had no cases.On 12 April, the North's official news state media KCNA reported that Kim had inspected drills by fighter jets at an air defence unit.There were no state media reports of any drills after the South said two days later that Pyongyang had launched a series of short-range cruise missiles. But analysts have said his absence from the 15 April commemorations suggested he could be looking to emphasise his own authority over his family's legacy.Some South Korean officials raised doubts about the credibility of the Daily NK 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.Analysts also expressed caution.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 an operation."Nothing is confirmed at this point, and it's too early to make a conclusion about his condition," he added.