China and South Korea cast doubt on Tuesday on reports North Korean leader Kim Jong Un was gravely ill after media outlets said he underwent a cardiovascular procedure.

Speculation over the state of his health was fuelled by his absence from a key anniversary event with the Daily NK, a speciality website run mostly by North Korean defectors, on Tuesday citing unidentified sources in the isolated state saying Kim was recovering at a villa in the Mount Kumgang resort after surgery on April 12.

CNN, citing a senior US official with knowledge of the situation, then reported Kim, 36, was in "grave danger".

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Kim's potential health issues could fuel uncertainty over the future of the reclusive state's dynastic rule and stalled denuclearisation talks with the United States, issues in which Kim wields absolute authority.

Two South Korean government sources told Reuters news agency the reports Kim was in "grave danger" were not true, while the presidential Blue House said there were no unusual signs coming from the north.

China said on Tuesday it is aware of reports about Kim's health but said it did not know their source, without commenting on whether it has any information about the situation. Chinese foreign ministry spokesman Geng Shuang made the remarks during a daily briefing and did not elaborate further.

An official at the Chinese Communist Party's International Liaison Department, which deals with North Korea, said he did not believe Kim was critically ill.

North Korea marked the anniversary of the birthday of its founding father and Kim's grandfather, Kim Il Sung, as a national holiday on April 15, but there was no sign of Kim in the images shared by the state news agency.

Kim Jong Un in pictures released in early December during a visit to battle areas of Mount Paektu [File: KCNA via Reuters]

Reporting from North Korea is notoriously difficult, especially on matters concerning the country's leadership.

Chad O'Carroll, the chief executive of Korea Risk Group, which produces the respected NK Pro and NK News, sounded a note of caution.

"It IS definitely notable that Kim Jong Un was missing all last week given the 04.15 holidays," he wrote on Twitter. "But it should be acknowledged that there are *many* rumours about his health going on at the moment. Some even regurgitate stuff we heard when KJU disappeared in 2014."

He noted Kim had appeared in public three times in January and February and six times in March. So far in April, he has also appeared three times.

Overweight, smoker

Kim is heavily overweight and his health has deteriorated in recent months because of heavy smoking and overwork, the Daily NK said.

"My understanding is that he had been struggling [with cardiovascular problems] since last August but it worsened after repeated visits to Mount Paektu," a source was quoted as saying, referring to the country's sacred mountain.

Can't yet confirm any of the speculation about KJU. We do know this: 1) the NK leader has vascular problems; 2) he missed 4/15. All other absences are explainable. Unless we get second and third sources on the story, we're still in the dark, and could be for a while.... — Victor Cha (@VictorDCha) April 21, 2020

Kim went into hospital after presiding over a meeting of the governing Workers' Party's Politburo on April 11, his last significant public event, the report said.

Pyongyang fired multiple short-range missiles last week which Seoul officials said were also part of the Kim Il Sung birthday celebration. Such military events would usually be observed by Kim, but there was no KCNA report on the test at all.

Kim is a third-generation hereditary leader who rules North Korea with an iron-fist, taking over the titles of head of state and commander in chief of the military since late 2011.

In recent years Kim has launched a diplomatic offensive to promote both himself as a world leader, holding three meetings with US President Donald Trump, four with South Korean President Moon Jae-in, and five with China's President Xi Jinping.

He was the first North Korean leader to cross the border into South Korea to meet Moon in 2018. Both Korea's are technically still at war, as the Korean War of 1950-53 ended in an armistice, not a peace treaty.

Kim has sought to have international sanctions against his country eased, but has refused to dismantle his nuclear weapons programme, a steadfast demand by the United States.