Kim Jong-un's disappearance from the public eye has raised speculation about the North Korean leader's health as well as who would take over should anything happen to him.

North Korea is one of the world's most isolated and secretive countries, and the health of its leaders is treated as a matter of state security. Reuters has not been able to independently confirm any details on Kim's whereabouts or condition.

Kim, believed to be 36, has disappeared from coverage in North Korean state media before. In 2014, he vanished for more than a month and North Korean state TV later showed him walking with a limp. Speculation about his health has been fanned by his heavy smoking, apparent weight gain since taking power and family history of cardiovascular problems.

Here is what we know — and don't know — about Kim's health:

When was Kim last seen?

North Korea's state media last reported on Kim's whereabouts when he presided over a meeting on April 11. According to the North's official Korean Central News Agency, he discussed coronavirus prevention at the meeting and elected his sister, Kim Yo-jong, as an alternate member of the political bureau of the ruling Workers' Party.

Questions about Kim Jong-un's health flared after he skipped an April 15 commemoration of the 108th birthday of his late grandfather, North Korea founder Kim Il-sung. Kim hadn't missed the event since inheriting power from his father, Kim Jong-il, in late 2011.

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Report of heart procedure

Daily NK, a Seoul-based website, reported on Monday that Kim Jong-un was recovering after undergoing a cardiovascular procedure on April 12. It cited one unnamed source in North Korea.

A special train possibly belonging to Kim was spotted this week at Wonsan, a North Korean resort town, according to satellite images reviewed by a Washington-based North Korea monitoring project.

The monitoring project, 38 North, said in its report on Saturday that the train was parked at the "leadership station" in Wonsan on April 21 and April 23. The station is reserved for the use of the Kim family, it said.

Though the group said it was probably Kim Jong Un's train, Reuters has not been able to confirm that independently, or whether he was in Wonsan.

"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," the report said.

On April 23, the train appeared to be repositioned for departure. However, there was no indication when that departure might take place, 38 North said.

Kim's health has deteriorated in recent months due to heavy smoking, obesity and overwork, a Daily NK report from Wednesday 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.

Not a 'poster boy for healthy living'

Kim left for the hospital after presiding over the April 11 meeting, the report said.

"I think it's entirely plausible that Kim Jong-un may have had an operation, or have had some sort of [other] health issue, because it's very unlikely for the leader of North Korea to miss Kim Il-sung's birthday on April 15," said North Korea expert Tina Park, co-founder and executive director of the Canadian Centre for the Responsibility to Protect at the University of Toronto and vice-president of the NATO Association of Canada.

"He missed it and the public would have noticed it and all the experts agree that that is a very unusual kind of movement on the part of North Korean leadership."

North Korea watchers also note that the dictator is not known for his healthy habits.

"Kim Jong-un is not exactly the poster boy for healthy living," said Marius Grinius, an Asia-Pacific security expert and retired director general of international security policy for Canada's Department of National Defence.

"If anything, he's getting fatter and fatter. Apparently he's a heavy smoker. And as any dictator, there's quite a bit of stress on him. And so he disappears and people start really wondering what's happening."

Any comment from North Korea?

The state-controlled media in North Korea has been silent on Kim's whereabouts, while South Korean officials reported no unusual activity in North Korea on Tuesday following the unconfirmed media reports.

North Korea's state media on Wednesday said Kim sent a message thanking Syrian President Bashar al-Assad for conveying greetings on his grandfather's birthday, but didn't report any other activities, while rival South Korea repeated that no unusual developments had been detected in the North.

A special train possibly belonging to Kim is seen in a satellite image with graphics taken over Wonsan, North Korea, on April 21, according to a Washington-based North Korea monitoring project. (Maxar Technologies-38 North/Handout/Reuters)

Danny Russel, a former U.S. National Security Council director and assistant secretary of state for Asia who has dealt with North Korea in the past, cautioned that rumours have abounded for years about Kim, his father and his grandfather, and most turned out to have been false.

"While serving in government I was on the receiving end of multiple intelligence reports about alleged accidents, illnesses and assassination attempts against North Korean leaders — only to have them reappear in public," he said.

'I hope he's OK,' Trump says

On Thursday, U.S. President Donald Trump downplayed earlier reports that Kim was gravely ill.

"I think the report was incorrect," Trump told reporters, but he declined to say if he had been in touch with North Korean officials.

Trump held unprecedented summits with Kim in 2018 and 2019 as part of a bid to persuade him to give up North Korea's nuclear arsenal.

U.S. President Donald Trump, right, last met with Kim in 2019 at the demilitarized zone separating North and South Korea in Panmunjom, South Korea. (KCNA/Reuters)

On Friday, a South Korean source told Reuters their intelligence was that Kim was alive and would likely make an appearance soon. The person said he did not have any comment on Kim's current condition or any Chinese involvement.

An official familiar with U.S. intelligence said that Kim was known to have health problems but they had no reason to conclude he was seriously ill or unable eventually to reappear in public.

Any comment from South Korea?

South Korean government officials, as well as a Chinese official with the Liaison Department, challenged reports suggesting that Kim was in grave danger after surgery.

"We have no information to confirm regarding rumours about Chairman Kim Jong-un's health issue that have been reported by some media outlets," South Korean presidential spokesperson Kang Min-seok said. "Also, no unusual developments have been detected inside North Korea."

At a closed door forum on Sunday, South Korea's Unification Minister Kim Yeon-chul, who oversees engagement with the North, said the government has the intelligence capabilities to say with confidence that there was no indications of anything unusual.

The South Korean presidential office later said Kim is believed to be staying at an unspecified location outside of Pyongyang, North Korea's capital, with some close confidants. It said Kim appeared to be normally engaged with state affairs, and there wasn't any unusual movement or emergency reaction from North Korea's ruling party, military or cabinet.

China sends medical experts

China has dispatched a team to North Korea including medical experts to advise on Kim, according to three people familiar with the situation. Reuters was unable to immediately determine what the trip by the Chinese team signalled in terms of Kim's health.

A delegation led by a senior member of the Chinese Communist Party's International Liaison Department left Beijing for North Korea on Thursday, two of the people said. The department is the main Chinese body dealing with neighbouring North Korea.

The sources declined to be identified given the sensitivity of the matter.

Last year, Chinese President Xi Jinping made the first state visit in 14 years by a Chinese leader to North Korea, an impoverished state that depends on Beijing for economic and diplomatic support. China is North Korea's chief ally and the economic lifeline for a country hard-hit by UN sanctions, and has a keen interest in the stability of the country with which it shares a long, porous border.

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Who would take over if something happened to Kim?

Kim is the third generation of his family to rule North Korea, and a strong personality cult has been built around him, his father and grandfather. The family's mythical "Paektu" bloodline, named after the highest peak on the Korean Peninsula, is said to give only direct family members the right to rule the nation.

That makes Kim's younger sister, senior ruling party official Kim Yo-jong, the most likely candidate to step in if her brother is gravely ill, incapacitated or dies.

"Among the North's power elite, Kim Yo-jong has the highest chance to inherit power, and I think that possibility is more than 90 per cent," said analyst Cheong Seong-Chang at the private Sejong Institute in South Korea. "North Korea is like a dynasty, and we can view the Paektu descent as royal blood, so it's unlikely for anyone to raise any issue over Kim Yo-jong taking power."

Kim Yo-jong, the North Korean leader's sister, is a likely candidate to step in if her brother is gravely ill, incapacitated or dies. (Jorge Silva/File/Reuters)

Sung-Yoon Lee, a professor of Korean Studies at Tufts University in Burlington, Mass., said Kim Yo-jong has "elevated her stature in recent weeks."

"In March, for example, for the first time under her own name she issued a statement slamming the South Korean president, calling him a scared dog, an idiot," he told CBC Radio's The Current.

"That suggests that her position within the North Korean political structure is unassailable, she is the number-two person."

Tina Park disagreed, saying that despite Kim Yo-jong's growing influence in recent years, "she's a very unlikely candidate as the next leader of North Korea."

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"From the eyes of people in leadership positions in North Korea, in the army or in political circles, she's seen as a baby. Because she's only in her 30s and a lot of them are in their 60s, and have dealt with previous dictators," Park said.

"From their perspective, she is not their equal, and they would be very uncomfortable having a young female leader as head of North Korea."

Lee said that Kim Yo-jong "will have to be ruthless" if she does succeed her brother.

"She may very well prove to be more tyrannical than her brother or their father or grandfather, because regime preservation in such a brutal totalitarian system demands it, demands ruthlessness," he said.

Other relatives

Kim Jong-chol is the leader's older brother but has not been part of the country's leadership, instead leading a quiet life playing music, according to Thae Yong-ho, North Korea's ex-deputy ambassador in London, who defected to South Korea.

He is believed to be disinterested in public life and is unlikely to emerge as a major presence, though some analysts say he maintains ties with siblings and could play a more public role in a contingency.

Kim Kyong-hui was once a powerful figure in the leadership circle when her brother Kim Jong-il ruled the country. But she has not been seen since her husband, Jang Song-thaek, once regarded as the second most powerful man in the country, was executed in 2013 by Kim Jong-un. She has long been ill but briefly appeared early this year at a gala performance alongside her nephew.

But some experts say a collective leadership, which could end the family's dynastic rule, could also be possible.

"North Korean politics and the three hereditary power transfers have been male-centred. I wonder whether she [Kim Yo-jong] can really overcome bloody socialist power struggles and exercise her power," said Nam Sung-wook, a professor at Korea University in South Korea.

A collective leadership would likely be headed by Choe Ryong-hae, North Korea's ceremonial head of state who officially ranks No. 2 in the country's current power hierarchy, Nam said. But Choe is still not a Kim family member, and that could raise questions about his legitimacy and put North Korea into deeper political chaos, according to other observers.

Other Kim family members who might take over include Kim Pyong-il, the 65-year-old half-brother of Kim Jong-il, who reportedly returned home in November after decades in Europe as a diplomat.