South Korean officials have played down rumours North Korean leader Kim Jong-un may be ill. It comes after reports he may have undergone heart surgery. "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," Moon Chung-in, the top foreign policy adviser to South Korean President Moon Jae-in, said. At a closed door forum on Sunday, South Korea's Unification Minister Kim Yeon-chul, who oversees engagement with North Korea, said Seoul was confident there was nothing unusual happening regarding Mr Kim. Speculation about the North Korean leader's health began after he was not seen publicly during an important state holiday on April 15, or in the weeks since then. Last week, South Korean media reported Mr Kim may have undergone cardiovascular surgery or was in isolation to avoid exposure to coronavirus. There were no new images of Mr Kim on North Korean state media on Monday, nor did outlets report on his whereabouts. However, the country's media carried reports that Mr Kim sent a message of gratitude to workers building a tourist resort in Wonsan, an area where some South Korean media reports said Mr Kim may be staying. Satellite images of Wonsan from last week showed a special train possibly belonging to Mr Kim, lending weight to those reports, according to 38 North, a North Korea monitoring project in Washington, DC. Though the group said the train probably belonged to the North Korean leader, there was no independent confirmation whether he was in Wonsan. “The train’s presence does not prove the whereabouts of the North Korean leader or indicate anything about his health, but it does lend weight to reports that Kim is staying at an elite area on the country’s eastern coast,” the 38 North report said. Last week, China dispatched a team to North Korea, including medical experts, but it was unclear what the trip by the Chinese team signalled in terms of Mr Kim's health. On Friday, a South Korean source told Reuters the country's intelligence suggested Mr Kim was alive and would likely make an appearance soon. Experts said Mr Kim disappeared from state media coverage before and that gathering accurate information in North Korea was difficult. North Korea's state media last reported on Mr Kim's whereabouts when he presided over a meeting on April 11. Mr Kim, who is believed to be 36, vanished from state media for more than a month in 2014 and North Korean state TV later showed him walking with a limp.