US and South Korean intelligence believe that reports of North Korean leader Kim Jong Un's death are untrue, The Washington Post reported.

Rumors have swirled of Kim's illness and death since he missed celebrations marking his grandfather's birthday on April 15.

A report from a Washington DC-based monitoring group on Saturday said that Kim's personal train had been pictured at an elite North Korean coastal resort last week.

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US intelligence agencies do not believe that reports spreading across the internet of North Korean leader Kim Jong Un's death are accurate, The Washington Post reported.

Three government officials familiar with the matter told the publication that US and South Korean intelligence believe that Kim is still alive.

Satellite images from a Washington-based North Korean monitoring project, 38 North, showed the leader's train last week at Wonsan, a coastal resort in the country. "We understand that Chairman Kim Jong Un has been in Wonsan this week," a South Korean intelligence official told the publication.

Rumors have been swirling about Kim's health since he missed the April 15 public events celebrating the birth of his grandfather, who founded the regime Kim now presides over.

North Korean state media reported that he attended a military review and government meetings days earlier, but since then his whereabouts have been a mystery.

On Saturday, he missed North Korea's Military Foundation Day that honors Kim's grandfather, fuelling a fresh wave of speculation.

38 North said in a statement that the leader's train was photographed at Wonsan on April 21 and April 23.

"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 38 North report said.

A senior Pentagon official told Newsweek that Kim's absence from public events and the presence of the train at the resort lent credibility to rumors he was ill. On Friday. Reuters reported that China had dispatched a group of doctors to North Korea to advise on Kim's health.

The news agency said it was "unable to immediately determine what the trip by the Chinese team signaled in terms of Kim's health."