US President Donald Trump said on Monday he has a good idea how Kim Jong Un is doing and hopes he is fine, after days of speculation over the North Korean leader's health.

Kim's whereabouts and whether he had a heart procedure have been a subject of intense speculation in recent weeks. Daily NK, a Seoul-based website, reported last week that Kim was recovering after undergoing a cardiovascular procedure on April 12, citing one unnamed source in North Korea. Reuters has not been able to confirm the report.

A special train possibly belonging to Kim was spotted last week at the North Korean resort town of Wonsan, according to satellite images reviewed by 38 North, a Washington-based North Korea monitoring project.