North Korean leader Kim Jong Un is understood to be "extremely obese", Seoul's spy agency said Wednesday, citing its latest assessment of the reclusive leader's health.

Axar.az reports, citing AFP, a heavy smoker who has long been obese, Kim's health is closely watched internationally as his sudden death could raise questions over succession and stability in the nuclear-armed state.

Kim's "health condition is classified as extremely obese, weighing over 140 kilogrammes (about 310 pounds)," lawmaker Youn Kun-young said citing Seoul's spy agency.

While he is at "high risk of cardiovascular disease", Kim has shown no difficulty with physical activity, including running up stairs, Youn added.

"It is assessed that there are no signs of health issues, given the close monitoring by (his) dedicated medical team."

Kim's father Kim Jong Il and grandfather Kim Il Sung were also obese and heavy smokers. Both died of heart attacks.