North Korean state media on Wednesday made no mention of new appearances by leader Kim Jong Un, a day after intense international speculation over his health was sparked by media reports he was gravely ill after a cardiovascular procedure.

South Korean and Chinese officials and sources familiar with US intelligence have cast doubts on the South Korean and US media reports, while the White House said it was closely monitoring the matter.

US President Donald Trump, who held unprecedented summits with Kim in 2018 and 2019 in an attempt to persuade him to give up his nuclear weapons, said the reports had not been confirmed and he did not put much credence in them.

"I just hope he's doing fine," Trump told a White House news conference on Tuesday. "I've had a very good relationship with Kim Jong Un. And I'd like to see him do well. We'll see how he does. We don't know if the reports are true."

Asked whether he would try to reach out to Kim to check on his condition, Trump said: "Well I may, but I just hope he's doing fine."

Speculation about Kim's health first arose due to his absence from the anniversary of the birthday of North Korea's founding father and Kim's grandfather, Kim Il Sung, on April 15.

On Wednesday, the main headlines from KCNA included pieces on sports equipment, mulberry picking, and a meeting in Bangladesh to study North Korea's "juche" or self-reliance ideology. Official Rodong Sinmun newspaper carried articles on a self-sufficient economy and anti-coronavirus measures.

There was no mention of Kim's whereabouts.

Daily NK, a Seoul-based website, reported late on Monday that Kim, who is believed to be about 36, was hospitalised on April 12, hours before the cardiovascular procedure.

It said his health had deteriorated since August due to heavy smoking, obesity and overwork.

Citing one unnamed North Korean source it said Kim was now receiving treatment at a villa in the Mount Myohyang resort north of the capital Pyongyang.

On Tuesday, CNN reported an unnamed US official saying that the United States was "monitoring intelligence" that Kim was in grave danger after surgery.

However, two South Korean government officials rejected the CNN report and South Korea's presidential Blue House said there were no unusual signs from North Korea. China, North Korea's only major ally, also dismissed the reports.

Trump's national security adviser, Robert O'Brien, told Fox News the White House was monitoring the reports "very closely".

"There's lots of conjecture going around," a senior Trump administration official said on condition of anonymity late on Tuesday when asked if there was confirmation of the reports.

North Korea experts have cautioned that hard facts about Kim's condition are elusive but said his unprecedented absence from major celebrations for his grandfather's birthday last week signals that something may have gone awry.

Kim is a third-generation hereditary leader who rules North Korea with an iron fist, coming to power after his father Kim Jong Il died in 2011 from a heart attack.

Reporting from inside North Korea is notoriously difficult, especially on matters concerning its leadership, given tight controls on information. There have been past false reports regarding its leaders, but the fact Kim has no clear successor means any instability could present a major international risk.

Trump said he had asked Kim about succession in the past but declined to elaborate.

"The basic assumption would be maybe it would be someone in the family," said O'Brien. "But, again, it's too early to talk about that because we just don't know what condition Chairman Kim is in and we'll have to see how it plays out."

With no details known about Kim's young children, analysts said Kim's sister and loyalists could form a regency until a successor is old enough to take over.

In recent years, Kim has launched a diplomatic offensive to promote himself as a world leader, holding three meetings with Trump, four with South Korean President Moon Jae-in and five with Chinese President Xi Jinping.

Kim has sought to have international sanctions against his country eased, but has refused to dismantle his nuclear weapons programme, a steadfast demand by the United States.

- Reuters