Beijing: North Korean leader Kim Jong-un has been detected by US sources walking in the port city of Wonsan, days after rumours spread that he was gravely ill after surgery.

South Korean newspapers have published accounts from sources who claim the leader was inspecting construction of the city's designated tourist area - a sprawling resort complex in the old trading port.

North Korean leader Kim Jong-un. AP

CNN reported on Tuesday the US was monitoring intelligence that Kim was "in grave danger" following surgery at his family's Hyangsan Clinic in North Pyongan province.

The Daily NK, a website run by North Korean defectors in Seoul, which maintains sources in the notoriously secretive regime, said excessive smoking, obesity, and overwork were the direct causes of Kim's urgent treatment and that he had been struggling with cardiovascular disease since August last year.

South Korean news outlet Joongang Ilbo reported speculation from northern sources that Kim had recently had a stent operation.

Business as usual. Visitors near the border between North and South Korea add to messages and flags of re-unification hope. There is still no official word on Kim's health. AP

A source told the outlet the dictator's residence had its own medical facilities and equipment: "How would North Korea not prepare for that as Kim Il-sung and Kim Jong-il died from a heart attack?"

Another source said: "Chairman Kim is always accompanied by a dedicated physician."

In Washington, US President Donald Trump also dismissed reports Kim was seriously ill, but would not say if he had been in touch with North Korean officials.

"I think the report was incorrect," Trump said at a daily White House briefing, adding that he had heard it was based on "old documents".

"We have a good relationship with North Korea, I have a good relationship with Kim Jong-un and I hope he's OK."

with Reuters