South Korea has said that Kim Jong-un, is “alive and well”, downplaying rumours that that the North Korean leader was seriously ill after undergoing heart surgery.

“Our government position is firm,” Moon Chung-in, a special adviser on national security to the South Korean president, Moon Jae-in, said in an interview with CNN on Sunday. “Kim Jong-un is alive and well.”

The adviser said that Kim had been staying in Wonsan – a resort town on the country’s east coast – since 13 April, adding: “No suspicious movements have so far been detected.”

Conjecture about Kim’s health has grown since his conspicuous absence from the 15 April celebrations for the birthday of his grandfather Kim Il-sung, the North’s founder – the most important day in the country’s political calendar.

Kim has not made a public appearance since presiding over a Korea Workers’ party politburo meeting on 11 April. His absence has unleashed a series of unconfirmed media reports over his condition, but officials in Seoul and Beijing have insisted that there is nothing to suggest Kim is unwell.

“We have nothing to confirm and no special movement has been detected inside North Korea as of now,” the South’s presidential office said in a statement last week.

Speculation about Kim grew after Daily NK, a Seoul-based online media outlet that employs North Korean defectors, claimed he was recovering after undergoing a “cardiovascular procedure” earlier this month.

Citing a single unidentified source inside the country, it said Kim, who is believed to be 36, had required urgent treatment due to heavy smoking, obesity, and fatigue.

Soon afterward, CNN reported that Washington was “monitoring intelligence” that Kim was in “grave danger” after undergoing surgery, quoting what it said was an anonymous US official.

The US president, Donald Trump, on Thursday rejected reports that Kim was ailing but declined to state when he was last in contact with him.