Kim Jong-un disappearance: South Korean spy agency says leader had Tarsal Tunnel Syndrome

South Korea's spy agency believes it has solved the mystery of Kim Jong-un’s 40 day disappearance – and it does not involve gout, an attempted coup or an addiction to Swiss cheese.

Instead, the National Intelligence Service (NIS) believes a foreign doctor operated on the leader of North Korea to remove a cyst from his right ankle because he was suffering from Tarsal Tunnel Syndrome.

Kim is believed to have been operated on by doctors in September or October.

Park Byeong-seok, an aide for opposition politician Shin Kyung-min said the spy agency told legislators in a closed-door briefing the condition could recur of Kim's obesity, smoking and heavy public schedule.

Lim Dae-seong, an aide to ruling parliamentarian Lee Cheol-woo, who also attended the briefing, said the spy agency identified Kim's condition as Tarsal Tunnel Syndrome.

The syndrome is caused by the compression of a nerve, sometimes because of a cyst. Surgery is generally considered a last resort after other treatments are unsuccessful.

After last being seen in state media on 3 September, Kim reappeared on 14 October, walking with a cane. He was seen hobbling in his final public appearance before disappearing from view, prompting speculation among some that his love of Swiss cheese had led to gout.

The speculation over his absence was particularly intense because of the Kim family's importance to impoverished, nuclear-armed North Korea. The family has ruled the country since its founding in 1948 and Kim is invariably at the centre of all state propaganda.

The agency also said North Korea has expanded five of its political prisoner camps, including the Yodok camp, which was relocated to the north-west city of Kilchu, according to Mr Lim. He said it believes the camps hold about 100,000 prisoners.

Mr Lim said the agency believes North Korea recently used a firing squad to execute several people who had been close to Kim's uncle, Jang Song-thaek, who was killed in a December 2013 purge.

The Yonhap news agency quoted the NIS as saying the notoriously secretive state has executed an estimated 50 people this year and demoted dozens of senior military officers.

It was not immediately clear how the information was obtained by the spy agency.

Additional reporting by agencies