South Korea and China have played down speculation that Kim Jong-un is seriously ill, after a Seoul-based website reported that the North Korean leader had undergone heart surgery.

Daily NK claimed Kim, who has not been seen in public for 10 days, was being treated at a private villa following the procedure this month.

CNN, on the other hand, cited an anonymous US official as saying that Washington was "monitoring intelligence" suggesting that Kim was in "grave danger".

But Kang Min-seok, a spokesman at South Korea’s presidential Blue House, said there was "nothing to confirm rumours about chairman Kim Jong-un’s health, and no special movement has been detected inside North Korea as of now."

Speculation about Kim’s health grew after he missed an event to mark the anniversary of the birth of his grandfather – and the country’s founder – Kim Il-sung – on 15 April.

Daily NK claimed that Kim had been admitted to hospital after chairing a meeting of the ruling workers’ party’s politburo on 11 April – this was his most recent public appearance.