Rumours are intensifying about Kim Jong-un's condition as a Chinese team including health care experts is dispatched into North Korea to treat him, Reuters reports.

The trip by Chinese doctors and officials to Pyongyang comes after reports the North Korean leader was in a critical condition after cardiovascular surgery.

A delegation including the medical staff and led by a senior member of the Chinese Communist Party's International Liaison Department left Beijing for North Korea on Thursday.

Earlier in the week, South Korean news site Daily NK reported Kim was recovering after undergoing a cardiovascular procedure on April 12.

US officials said they were told Kim's condition was critical after the surgery, although President Donald Trump has since cast doubt on the report.

Seoul officials and a source within the Liaison Department have challenged reports Kim was gravely ill.

Speculation about Kim's condition has escalated in the two weeks since he was last seen in public, at a politburo meeting on April 11.

On April 15, he was conspicuous in his absence from birthday celebrations of the founder of the North Korean state, his grandfather and state founder Kim Il Sung.

On Thursday, President Trump told reporters he thought the report Kim was gravely ill "was incorrect" but declined to say if he had been in touch with North Korea.

US Secretary of State Mike Pompeo told Fox News when asked about Kim's health, "I don't have anything I can share with you tonight, but the American people should know we're watching the situation very keenly".

Kim Jong-un, whose age is either 36 or 37, is said to be a heavy smoker, and has gained weight since he succeeded his father in 2011.

He also may have hereditary cardiovascular disease.

In 2008, his father Kim Jong-il suffered a stroke and was reportedly treated by Chinese and French medical specialists.

He died in 2011 from a heart attack at the age of 70.

North Korea's founder Kim Il Sung, who the current Kim closely resembles, ruled the country until his death in 1994, aged 82.

Donald Trump made history when he met with Kim Jong-un, in Singapore in 2018, for the first-ever talks held between a North Korean leader and a sitting US president.