A special train believed to belong to Kim Jong-un, the North Korean leader, was spotted this week at a coastal resort, boosting speculation that he may be staying there while being treated for reported health problems.

Satellite images reviewed by the Washington-based North Korea monitoring project 38 North, appeared to show that a train similar to Kim’s was parked in the so-called “leadership station” – reserved for the use of the Kim family - in Wonsan, on the east coast, on April 21 and 23.

The report comes amid wildly varying rumours that Kim, said to be 36, recently had cardiovascular surgery and was either recovering or possibly in “grave danger.”

Worst case scenarios have been played down this week by the South Korean government – who noted no unusual activity in North Korea - and Donald Trump, the US president.

China has dispatched a team to North Korea including medical experts to advise on Kim, three people familiar with the situation told Reuters.

“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 on Saturday.

The group revealed that the luxury Wonsan complex includes nine large guesthouses, a protected port, shooting range, recreation building and a covered dock that is believed to be used for Kim’s mega yacht. A nearby small runway had been converted into a horse-riding track to match Kim’s latest hobby.

If confirmed, the ability of Kim to travel around the country on his train might suggest that he is recuperating from illness rather than in a life-threatening condition. Dong-a-Ilbo, a South Korean newspaper, cited a US official in Washington saying the leader had been seen walking around in Wonsan.

"There is a lot we don't know about Kim Jong-un, but the one thing we do know is that he is dangerously overweight and unhealthy," said Richard McGregor, senior fellow with Australian think tank the Lowy Institute. "The Chinese would have been keen to send in a medical team, if for no other reason than to gather intelligence.

"The North Koreans are very suspicious of China and they are often as much in the dark on elite leadership developments as anyone else."

Kim’s location and health condition has been the subject of heated speculation since he failed to show up at events to celebrate the anniversary of the birthday of North Korean founding father, and his grandfather - Kim Il-sung on April 15.

Since he came to power in 2011, Kim has not missed the celebration of the Day of the Sun, one of the most important national holidays of the year.

Rumours he had missed the event for health reasons were kicked off by The Daily NK, a new site founded by South Korean human rights and democracy activists, which cited an anonymous source as saying that Kim had undergone a “cardiovascular surgical procedure” on April 12 and was mostly recovered.

This was followed by a more alarming claim by an unnamed US official who told CNN that the US was “monitoring intelligence” that the North Korean leader was in “grave danger” following a surgery.

President Trump later accused CNN of making the report up, but Robert O’Brien, the US national security adviser, confirmed that Washington is "keeping a close eye" on reports regarding Kim’s health.