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

North Korean media presented a business-as-usual image, carrying routine reporting of Kim's achievements and publishing some of his older, or undated, comments on issues like the economy.

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

However, on Wednesday one US government source who had previously played down reports that Kim was seriously ill said it was a possibility that was now being looked at closely.

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 on Tuesday the reports had not been confirmed and he did not put much credence in them.

"We'll see how he does," Trump told a White House news conference. "We don't know if the reports are true."

US Secretary of State Mike Pompeo said on Wednesday he had nothing to add to Trump's remarks. "I think it was, we're watching closely what's taking place there," Pompeo told a news conference.

The second-highest ranking US general said he had no intelligence to "confirm or deny" reports about Kim's health and he assumed he was still in control of North Korea's military.

"I have no reason not to assume that," Air Force General John Hyten, vice chairman of the Joint Chiefs of Staff, told a news conference.

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 North Korea's state news agency, KCNA, included pieces on sports equipment, mulberry picking, and a meeting in Bangladesh to study North Korea's "juche" or self-reliance ideology.

The official Rodong Sinmun newspaper carried older or undated remarks attributed to Kim in articles about the economy, the textile industry, city development, and other topics.

Official media has however continued to report the sending of routine diplomatic letters by Kim, and KCNA said he sent a reply on Wednesday to Syrian President Bashar al-Assad, thanking him for a message to mark the birthday of the North Korean leader's grandfather, Kim Il Sung.

South Korea's presidential Blue House said it could not confirm Kim's whereabouts, or whether he had undergone surgery. It said South Korea had detected no unusual activity in North Korea.

It said Kim's health had deteriorated since August due to heavy smoking, obesity and overwork, and he was now receiving treatment at a villa north of Pyongyang.

States was "monitoring intelligence" that Kim was in grave danger after surgery.

However, two South Korean government officials rejected the CNN report. China, North Korea's only major ally, also dismissed the reports.