Kim Jong-un is not ill and could be sheltering from the coronavirus pandemic, according to South Korean and US officials, in the latest possible explanation for the North Korean leader’s recent absence from public life.

North Korea insists it has yet to identify a single case of Covid-19, despite sharing a border with China, where the outbreak is believed to have started.

Despite recent reports claiming that Kim had missed a key anniversary event after undergoing heart surgery, the South Korean unification minister, Kim Yeon-chul, said it was reasonable to assume that he had decided not to attend as a precaution.

Kim was absent from a 15 April ceremony to mark the anniversary of the birth of his grandfather – and North Korea’s founder – Kim Il-sung.

“We have intelligence capacity that allows us to say confidently that there are no unusual signs [in North Korea],” Kim Yeon-chul told a parliamentary hearing.

“He had not missed the anniversary … since taking power, but many anniversary events, including celebrations and a banquet, have been cancelled because of coronavirus concerns.

“I don’t think that’s particularly unusual given the current situation,” he said, describing reports that Kim had undergone heart surgery as “fake news”.

US officials told Reuters they believed Kim had taken his private train to the east coast resort town of Wonsan to shelter from the virus.

Kim Yeon-chul noted that there had been at least two instances since mid-January when Kim had not been seen in public for almost three weeks.

The North Korean leader was last seen in public on 11 April when he presided over a party politburo meeting. State media have not published a single photograph of Kim since, but they have carried reports of his daily routine, including diplomatic messages sent to other world leaders.

Speculation that Kim was staying in Wonsan grew over the weekend after the Washington-based North Korea monitoring project 38 North said it had analysed satellite images showing his special train parked at the local station.

On Tuesday, the NK News website said leisure boats often used by Kim had been active in the area throughout this month.

North Korea sealed its border with China in the early stages of the coronavirus outbreak, quarantined thousands of people and scaled down public events. Observers are sceptical, though, of the regime’s claims that the country is free of the virus.

Edwin Salvador, the World Health Organization’s representative to North Korea, said last week that the country reported that it tested 740 people for Covid-19 as of 17 April but that all of them were negative. The North said it had so far released more than 25,000 people from quarantine since late December, according to Salvador.

Jiro Ishimaru, who heads the Osaka-based Asia Press website and operates a network of citizen journalists in North Korea, said confirmation that Kim was in isolation would prove that the virus had spread inside the country.

“I don’t think he is dead – if that was the case there would be a lot more identifiable movement,” Ishimaru, who has spoken recently to North Koreans near the Chinese border, told the Guardian. “One theory is that either he himself has the virus or people around him have it and he’s is sheltering.”

Ishimaru said it was possible that the regime had deliberately sent Kim’s train to Wonsan – knowing satellite images would be seen by analysts – to give the impression that he was simple taking a break when, in fact, he was self-isolating elsewhere.

“All of the attention is on Wonsan, but there is a chance that he is fooling everyone,” Ishimaru said.

Chad O’Carroll, the chief executive of Korea Risk Group, which monitors North Korea, said any evidence that Kim was sheltering from the virus would “puncture a hole in the state media narrative of how this crisis has been perfectly managed”.