South Korea's National Intelligence Service revealed Tuesday the reason behind North Korean leader Kim Jong Un's absence from public view for nearly six weeks - an ankle surgery.

In a closed door briefing by the spy agency, South Korean legislators were informed that Kim had surgery in September or October to remove a cyst from his ankle, said lawmaker Shin Kyung Min. Shin also said that a foreign doctor performed the operation on Kim and that there were chances of a cyst relapse due to Kim's heavy smoking, obesity and hectic public programs.

Kim has been diagnosed with tarsal tunnel syndrome, a painful situation caused by the compression of a nerve, Shin said, reports the Associated Press.

Surgery is done to cure the condition as other modes of treatment are most often ineffective.

Shin also revealed that  the agency believed that Kim put to death several people who were close to his  uncle Jan Song Tabek, who was the vice-chairman of the National Defence Commission, a position considered second only to that of the Supreme Leader.  Jang was also the main policy adviser to Kim.

According to Shin's office, the agency informed the lawmakers that North Korea had recently enlarged five of its political prison camps. The agency also thinks that around 100,000 prisoners are held in the camp.

Meanwhile, Marzuki Darusman, the United Nations' special rapporteur dealing with North Korea, said in a report published Tuesday that North Korea might be referred to the International Criminal Court on charges of crimes against humanity.

"The international community must seize this unique opportunity and momentum created by the commission of inquiry to help to make a difference in the lives of the people of [North] Korea, including victims, and to ensure accountability of those responsible for serious violations of human rights, including crimes against humanity," Darusman wrote in his report, published Tuesday, according to The Washington Times.

Darusman's statement comes six months after a U.N. commission of inquiry released a 372-page report which gave a detailed description on the crimes committed by the North Korean regime like torture, starvation and imprisonment for questioning the system or trying to escape it.