The younger sister of North Korean dictator Kim Jong-un has taken over important state duties from her ailing brother, as he recovers from a mysterious illness, it has been reported.

According to a Seoul-based think tank, Kim Yo-jong has been entrusted with handling crucial government powers by Pyongyang's top political body in September, after doctors said the Supreme Leader was in need of extended medical treatment to restore his health.

"Some say Hwang Byong So, director of the General Political Bureau of the Korean People's Army, may have assumed new No. 2 status, but given what has been confirmed this time, we can say Hwang is just a shadow, and Kim Yo-jong is the second-in-command of North Korea," Kim Heung Gwang, the head of North Korea Intellectuals Solidarity (NKIS) said, Japanese magazine The Diplomat reported.

Other decisions taken at the emergency meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea, which was reportedly called by Kim Yo-jong herself, included upholding orders issued by the dictator before he fell ill and placing the army on wartime-like alert.

Kim Yo-jong, believed to be 26, was a shadowy figure before making her public debut as a government official in March, as she accompanied her elder brother at a polling satiation for parliamentary elections.

She was shown on North Korean state TV trailing her brother and party officials on the way to casting votes at Kim Il-sung University of Politics in Pyongyang.

Dressed in a black skirt suit, she was identified by commentators as "comrade Kim Yo-jong" and described as a senior official, a rank she has not previously been known to hold.

She had been earlier seen in tears at Kim Jong-il's funeral in 2011 and has appeared sporadically alongside her brother over the past few years.

Analysts said that her ascent in the party ranks signalled the increasing grip on power of Kim's family.

Meanwhile the reasons for Kim's sustained absence from the public scene continue to be the subject of speculation.

He was last seen on 3 September during a concert flanked by his wife. He was walking with a limp at the performance, sparking claims he has since undergone surgery for fractured ankles.

Other reports suggested the Supreme Leader has been suffering from gout.

However, reports from the hermit nation are impossible to confirm and often suffer from South Korean sensationalism.