Amid mounting rumors on North Korean leader Kim Jong-un's health, a special security adviser to South Korean President Moon Jae-in said on Sunday that Kim is "alive and well."

Moon Chung-in, special adviser on unification, diplomacy and security affairs, said during an interview with Fox News that his government's official position is firm that Kim is alive and well. 

The adviser said that Kim has been staying in the Wonsan area since April 13, adding no suspicious movements have so far been detected.

38 North, a U.S.-based North Korea monitoring site, said on Saturday that commercial satellite imagery indicated that a special train possibly belonging to the North Korean leader has been parked at a rail station in Wonsan since at least Tuesday.

Fox News quoted other sources in South Korea as saying that Kim did not die but he might be recovering from surgery. 

Speculation about Kim's health has mounted since he has not been seen publicly since April 11 and, for the first time, even missed an annual celebration to honor his grandfather on April 15.