SEOUL (AFP, REUTERS) - North Korean leader Kim Jong Un is "alive and well", a top security adviser to the South's President Moon Jae-in said, downplaying rumours over Mr Kim's health following his absence from a key anniversary.

"Our government position is firm," said President Moon's special adviser on national security Mr Moon Chung-in, in an interview with CNN on Sunday (April 26). "Kim Jong Un is alive and well."

The adviser said that Mr Kim had been staying in Wonsan - a resort town in the country's east - since April 13, adding: "No suspicious movements have so far been detected."

Conjecture about Mr Kim's health has grown since his conspicuous absence from the April 15 celebrations for the birthday of his grandfather Kim Il Sung, the North's founder - the most important day in the country's political calendar.

Mr Kim has not made a public appearance since presiding over a Workers' Party politburo meeting on April 11, and the following day, state media reported him inspecting fighter jets at an air defence unit.

His absence has unleashed a series of unconfirmed media reports over his condition, which officials in Seoul previously poured cold water on.

"We have nothing to confirm and no special movement has been detected inside North Korea as of now," the South's presidential office said in a statement last week.

South Korea's unification minister Mr Kim Yeon-chul reiterated on Monday that this remained the case, adding that the "confident" conclusion was drawn from "a complex process of intelligence gathering and assessment".

He cast doubt on reports of a surgery, arguing that the hospital mentioned in the reports did not have the capabilities for such an operation.

The comments came two years after Mr Kim and Mr Moon's first summit in the Demilitarised Zone that divides the peninsula.

Seoul marked this anniversary with a ceremony at the South's northernmost train station, seeking to highlight its commitment to a cross-border railway project.

But inter-Korean relations are largely frozen with talks between Washington and Pyongyang at a standstill, and there was no indication of any commemoration in the North.

GRAVE DANGER

South Korea media reported last week that Mr Kim may have undergone cardiovascular surgery or was in isolation to avoid exposure to the new coronavirus.

Still, Mr Yoon Sang-hyun, chairman of the foreign and unification committee in South Korea's National Assembly, told a gathering of experts on Monday that Mr Kim's absence from the public eye suggests "he has not been working as normally".

"There has not been any report showing he's making policy decisions as usual since April 11, which leads us to assume that he is either sick or being isolated because of coronavirus concerns," he said.

Daily NK, an online media outlet run mostly by North Korean defectors, reported last week that Mr Kim was recovering after undergoing a cardiovascular procedure earlier this month.

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

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

Last Thursday, United States President Donald Trump rejected reports that Mr Kim was ailing but declined to state when he was last in touch with him.

On Monday, the official Rodong Sinmun newspaper reported that Mr Kim had sent a message of thanks to workers on the giant Wonsan Kalma coastal tourism project.

It was the latest in a series of reports in recent days of statements issued or actions taken in Mr Kim's name, although none has carried any pictures of him.

Satellite images reviewed by 38North, a US-based think tank, showed a train probably belonging to Mr Kim at a station in Wonsan last week.

It cautioned that the train's presence did not "indicate anything about his health" but did "lend weight" to reports he was staying on the country's eastern coast.

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.

Mr Andrei Lankov, director of Korea Risk Group, said all reports should be "taken with a grain of salt", but noted that Mr Kim's disappearance was unusual. "So for some reason, he is not really capable to act publicly," Mr Lankov told AFP.

Previous absences from the public eye on Mr 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.