A train likely belonging to North Korean leader Kim Jong Un has been spotted at a resort town in the country's east, satellite photos reviewed by a US-based think tank showed, as speculation persists over his health.

The train was parked at a station reserved for the Kim family in Wonsan on April 21 and April 23, the respected 38North website said in a report published Saturday.

38North cautioned that 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," it said.

There has been growing conjecture about Kim's health since his conspicuous absence from the April 15 celebrations for the birthday of his grandfather Kim Il Sung, the regime's founder -- the most important day on the North Korean political calendar.

Kim has not made a public appearance since presiding over a meeting of the Workers' Party politburo on April 11 and inspecting drills by fighter jets at an air defence unit, which was reported by state media on April 12.

- Out of sight -

Daily NK, an online media outlet run mostly by North Korean defectors, has reported Kim underwent a cardiovascular procedure earlier this month and was recovering at a villa in North Pyongan province.

Citing an unidentified source inside the country, it said Kim, who is in his mid-30s, had needed urgent treatment due to heavy smoking, obesity and fatigue.

South Korea, which is still technically at war with the North, has played down the report.

CNN, quoting what it said was an anonymous US official, reported that Washington was "monitoring intelligence" that Kim was in "grave danger" after undergoing surgery.

But on Thursday, US President Donald Trump rejected reports that Kim was ailing.

"I think the report was incorrect," Trump told reporters, but declined to state when he was last in touch with him.

"We have a good relationship with North Korea, as good as you can have," he said.

Story continues

Trump has met Kim three times in historic summitry and has voiced admiration for him, although hopes have dimmed for reaching a comprehensive agreement.

Reporting from inside the isolated North is notoriously difficult, especially on anything to do with its leadership, which is among its most closely guarded secrets.

On Thursday, citing an unidentified government official, South Korean broadcaster SBS reported that Kim appeared to have been in Wonsan for at least the past four days and would soon return to the public eye.

The report added that the military was monitoring Kim's train, which had been seen in Wonsan, while his personal jet -- frequently used by Kim on his trips to Wonsan -- remained in Pyongyang.

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.