(CNN) -- South Korea continued to pour water on mounting speculation about the health of North Korea's leader Kim Jong Un, telling CNN he is "alive and well."
"Our government position is firm," Moon Chung-in, the top foreign policy adviser to South Korean President Moon Jae-in, told CNN. "Kim Jong Un is alive and well. He has been staying in the Wonsan area since April 13. No suspicious movements have so far been detected."
Questions were raised about Kim's well-being after he missed the celebration of his grandfather's birthday on April 15. He had been seen four days before that at a politburo meeting, according to North Korean state media, KCNA.
CNN reported earlier in the week that the United States is monitoring intelligence that Kim is "in grave danger after a surgery," according to a US official. Another US official told CNN Monday that the concerns about Kim's health are credible, but their severity is hard to assess.
It followed a report by Daily NK, an online newspaper based in South Korea that focuses on North Korea, that Kim reportedly received a cardiovascular procedure because of "excessive smoking, obesity, and overwork." It added that the leader is now receiving treatment in a villa in Hyangsan County.
After assessing that Kim's condition had improved, most of the medical team treating him returned to Pyongyang on April 19 while some of them remained to oversee his recovery, according to the news site. CNN is unable to independently confirm the report.
On Sunday though, North Korean state newspaper Rodong Sinmun said Kim sent thanks to workers who helped in remodeling the city of Samjiyon in North Korea. CNN cannot independently confirm the authenticity of Sunday's report and whether the note of thanks came directly from the leader.
It is not the first time North Korean media has reported on Kim's activities since news of his potential health issues emerged.
Train in Wonsan
North Korea tightly controls any information surrounding its leader, who is treated almost like a deity within the country. His absences from official state media often spark speculation and rumors about his health.
North Korea has no free press and is often a black hole when it comes to the country's leadership. Analysts are heavily reliant on scanning state media dispatches and watching propaganda videos for any semblance of a clue.
A website specializing in North Korean affairs published satellite images on Saturday that researchers said showed a train "probably belonging to Kim Jong Un" has been parked at a railway station serving Kim's luxurious Wonsan compound on the country's eastern coast since at least April 21.
The presence of the train does not prove DPRK leader is at the Wonsan compound but "does lend weight" to reports he is staying in the area, according to the article published by 38 North, which publishes analysis on North Korean affairs and is a project of the Henry L. Stimson Center, a Washington, D.C.-based think tank.
The website reported the station where the train is parked is "reserved for use by the Kim family." Satellite images indicate the train, not present in the area on April 15, arrived sometime before April 21 and continued to be at the location on April 23. By April 23, the train "appeared to be repositioned for departure," according to 38 North.
The Wonsan complex, according to 38 North, includes leisure facilities such as nine large guesthouses, a recreation center, a large building constructed after Kim came to power, a protected port, a shooting range, a recreation building, a covered dock, and a small runway converted in 2019 into a horse-riding track, a hobby of Kim's.
The beachfront area, where Kim spent summers as a child, is a place the North Korean leader is known to enjoy frequenting. Kim, however, often travels by plane or drives to Wonsan.