Speculation mounted on Saturday that North Korean leader Kim Jong Un may be dead or in grave condition after a surgery.

The rumors started after Kim did not appear at a celebration for one of the country's most celebrated holidays on April 15: the birthday of founder Kim Il Sung.

The accuracy of these reports remains unclear — the South Korean and Chinese governments have downplayed concerns, and United States intelligence has not yet confirmed anything.

Experts have previously noted that Kim Jong Un had certain health risks and could suffer cardiac issues due to his weight.

The rumors have sparked a flurry of questions about Kim Jong Un's line of succession, and who might take his place if he dies.

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Speculation mounted on Saturday over North Korean leader Kim Jong Un's health as he hasn't been seen in public since April 11 when he presided over a Politburo meeting.

It's fueled reports from multiple outlets citing sources in China and Japan that Kim, who is believed to be 36, is in grave condition after a cardiovascular surgery, or may even be dead.

The accuracy of these reports is unclear. The South Korean government, which monitors North Korea closely, downplayed the rumors on April 21 and said Kim appears to still be running the country.

"No unusual developments have been detected inside North Korea," South Korean presidential spokesman Kang Min Seok said.

Chinese officials have also downplayed rumors that Kim is critically ill. US intelligence is reportedly monitoring the situation, but President Trump said earlier this week the US had no reliable information regarding Kim's health.

The speculation over his health has also prompted questions over who might take over as North Korea's leader if Kim dies, an event carrying sweeping implications for an unpredictable regime that's outlasted the Soviet Union and stockpiled between 10 and 60 nuclear weapons.

Kim has a complicated family tree, and his three rumored children are too young to lead. But his sister, Kim Yo Jong, is considered to be a powerful figure who could run the country in the meantime.

How the rumors began, and how they've evolved since

The rumors about Kim's health first began on April 15, when he did not appear at a celebration commemorating the birth of Kim Il Sung, his grandfather and the founder of North Korea. The birthday is the country's most important holiday.

Then, the rumors intensified when a South Korean news outlet published a story reporting the North Korean leader had undergone heart surgery and was recovering at a villa outside Pyongyang, the nation's capital. The report relied on a single unnamed source inside North Korea.

Reuters reported on Friday that China dispatched a team of medical experts to travel to North Korea and advise on Kim's health, citing three sources. But the outlet was quick to note it wasn't able to determine what the development meant about the North Korean leader's health, and the Chinese government did not respond to Reuters' requests for comment.

Adding to the mystery were satellite images published Saturday by a Washington-based North Korea monitoring project, 38 North. The images showed what appeared to be Kim Jong Un's private train at a resort town in North Korea.

What is described by Washington-based North Korea monitoring project 38 North as a special train possibly belonging to North Korean leader Kim Jong Un is seen in a satellite image with graphics taken over Wonsan, North Korea April 21, 2020. Satellite image ©2020 Maxar Technologies-38 North/Handout via REUTERS.

The information about the train remains unconfirmed, and it's unclear Kim himself was on the train.

"The train's presence does not prove the whereabouts of the North Korean leader or indicate anything about his health but it does lend weight to reports that Kim is staying at an elite area on the country's eastern coast," 38 North said.

As with most news regarding the hermit kingdom, the reports are near-impossible to verify. The government tightly controls its media apparatus, and has refused to release even the most basic information about the Kim family — including Kim Jong Un's age, the existence of his children, and when he married his wife.

Kim Jong Un is at risk from obesity and cardiac health issues, experts say

Media outlets in North Korea, including the state-run Korean Central News Agency, have been silent so far about Kim's absence from public events.

North Korea watchers say that one of Kim's biggest risk factors was his poor health, particularly risks from cardiac issues due to his weight, per The Washington Post's Anna Fifield.

"The young leader looks like a heart attack waiting to happen and has clearly had health problems," Fifield wrote in her book, "The Great Successor."

Fifield also noted that the North Korean leader is a heavy smoker, and South Korean doctors she interviewed said Kim was in a weak physical state for a relatively young adult.

North Korean leaders have previously disappeared only to reappear at public events weeks later

Back in 2014, the North Korean leader disappeared for nearly five weeks, fueling rumors he suffered from gout, a severe hangover, or had been overthrown in a coup.

North Korean state TV later showed Kim walking with a cane, and announced the leader was "not feeling well." South Korean intelligence said at the time he had an ankle surgery.

The same happened for the country's previous ruler, Kim Jong Il. In 2008, he was publicly absent for several months, setting off a rumor mill on his whereabouts and medical condition. A French doctor later confirmed Kim Jong Il suffered a stroke, The New York Times reported.

Information around significant events in the country can take several days or weeks to make its way to the outside world. When Kim Jong Il died in 2011, it took time for intelligence agencies to confirm it — but only when the news was announced on state television two days later.

Both Kim's father and grandfather died of heart failure. They share another common thread: Newspapers prematurely killed them off several times over the years during their rule before they actually died, according to The Post.

It underscores the challenges of discerning fact from fiction about information trickling out of the world's most secretive state. But one thing remains clear about arriving at any conclusion on the state of Kim's health.

"None of us will know until either North Korea tells us, or he waddles back into view," Fifield wrote in The Post on Sunday.