Australian intelligence officials are treating reports of North Korean leader Kim Jong-un's ill health seriously, with concerns about the upheaval that would be created by a change of leader in the secretive and authoritarian state.

Australia is monitoring intelligence that Kim is in serious danger after undergoing cardiovascular surgery earlier this month, but has no definitive proof that he is in a vegetative state or is dead, according to senior government sources.

A Hong Kong broadcast network claimed over the weekend that Kim had died, citing a “very solid source", while a Japanese magazine reported that he was in “a vegetative state”.

A train possibly belonging to the North Korean leader was spotted at a resort town near the "leadership station" between April 21 and 23, according to satellite images reviewed by a Washington-based think tank which focuses on North Korea. The station is reserved for the use of Kim and his family, but there is no definitive proof the train was carrying the North Korean ruler.