Satellite imagery has found a train likely belonging to North Korean leader Kim Jong Un as speculation continues over his health, according to a website specialising in studies of the country.

North Korean authorities have said nothing to counter media reports that Mr Kim is unwell, prompting concerns about who is next in line to run a nuclear-armed country that has been ruled by the same family for seven decades.

The satellite photos provided by website 38 North and Airbus show the train has been parked at Mr Kim’s compound in Wonsan on his country’s east coast since at least April 21.

The photos do not provide any clarity over Mr Kim’s health, but they do confirm South Korean government intelligence that he is staying outside of the capital, Pyongyang.

Seoul has also said repeatedly that there have been no signs from the North that Mr Kim is unwell.

Questions about his health flared after he skipped an April 15 commemoration of the 108th birthday of his grandfather, North Korea founder Kim Il Sung.

The annual commemoration is North Korea’s most important event, and Mr Kim, 36, had not missed it since inheriting power from his father in late 2011.

Recent satellite photos show a train probably belonging to North Korean leader Kim Jong Un on the country’s east coast amid mounting speculation about his health. Credit: Airbus Defence & Space and 38 North, Pleiades©CNES 2020 Distribution Airbus DS/AP

North Korea’s state media on Wednesday said Mr Kim sent a message thanking Syria’s president for conveying greetings on his grandfather’s birthday, but did not report any other activities.

Rival South Korea repeated no unusual developments had been detected in the North.

Mr Kim has been out of the public eye for extended periods in the past, and North Korea’s secretive nature allows few outsiders to assert confidently whether he might be unwell, let alone incapacitated.

Still, questions about the North’s political future are likely to grow if he fails to attend forthcoming public events.

The train has been parked at the Leadership Railway Station servicing his Wonsan compound since at least April 21. Credit: Airbus Defence & Space and 38 North, Pleiades©CNES 2020 Distribution Airbus DS/AP

Mr Kim is the third generation of his family to rule North Korea, and a strong personality cult has been built around him, his father and grandfather.

The family’s mythical “Paektu” bloodline, named after the highest peak on the Korean Peninsula, is said to give only direct family members the right to rule the nation.

That makes Mr Kim’s younger sister, senior ruling party official Kim Yo Jong, the most likely candidate to step in if her brother is gravely ill, incapacitated or dies.

North Korea’s Kim Yo Jong, centre at rear, is the most likely to succeed her brother. Credit: Mike Egerton/PA

But some experts say a collective leadership, which could end the family’s dynastic rule, could also be possible.

A collective leadership would likely be headed by Choe Ryong Hae, North Korea’s ceremonial head of state who officially ranks No.2 in the country’s current power hierarchy, Mr Nam said.

But Mr Choe is still not a Kim family member, and that could raise questions about his legitimacy and put North Korea into deeper political chaos, according to other observers.

Other Kim family members who might take over include Kim Pyong Il, the 65-year-old half-brother of Kim Jong Il, who reportedly returned home in November after decades in Europe as a diplomat.