Kim ‘alive and well’: Seoul

North Korean leader Kim Jong Un is "alive and well", a top security adviser to the South's President Moon Jae-in said, downplaying rumours over Kim's health following his absence from a key anniversary.

Conjecture over Kim has grown since his conspicuous no-show at April 15 celebrations for the birthday of his grandfather Kim Il Sung, the North's founder -- the most important day in the country's political calendar.

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"Our government position is firm," said Moon's special adviser on national security Moon Chung-in, in an interview with CNN. "Kim Jong Un is alive and well."

The adviser said that Kim had been staying in Wonsan -- a resort town in the country's east -- since April 13, adding: "No suspicious movements have so far been detected."

Kim has not made a public appearance since presiding over a Workers' Party politburo meeting on April 11, and the following day state media reported on him inspecting fighter jets at an air defence unit.

Daily NK, an online media outlet run mostly by North Korean defectors, has reported Kim was undergoing treatment after a cardiovascular procedure earlier this month.

Citing an unidentified source inside the country, it said Kim, who is in his mid-30s, had needed urgent treatment due to heavy smoking, obesity and fatigue.