North Korea has dismissed Jang Song-thaek, the once powerful uncle of leader Kim Jong-un, for what it described as a string of criminal acts including corruption, womanising and drug-taking.

South Korea's spy agency last week said it believed Jang, long regarded as the second most powerful man in the secretive state, had been relieved of his posts in November.

The sacking on Monday means Pyongyang is undergoing its biggest leadership upheaval since the death in 2011 of former leader Kim Jong-il, the younger Kim's father.

Affected by the capitalist way of living, Jang committed irregularities and corruption and led a dissolute and depraved life.

KCNA, North Korea's state news agency.

"Jang and his followers committed criminal acts baffling imagination and they did tremendous harm to our party and revolution," the North's KCNA news agency said in a report following a meeting of the ruling Workers' Party politburo on Sunday.

The meeting decided to dismiss Jang from all his posts and expel him from the Workers' Party, KCNA said. Kim Jong-un attended and "guided" the meeting, it said.

KCNA listed a series of acts committed by Jang that it said led to the decision to remove him, including mismanagement of the country's financial system, corruption, womanising and abusing alcohol and drugs.

"Jang pretended to uphold the party and leader but was engrossed in such factional acts [such] as dreaming different dreams and involving himself in double-dealing behind the scene," KCNA said.

"Affected by the capitalist way of living, Jang committed irregularities and corruption and led a dissolute and depraved life," KCNA said.

Jang is married to Kim's aunt, the daughter of the North's founding leader Kim Il-sung, and was widely considered to be working to ensure his nephew firmly established his grip on power in the past two years.

Last week a South Korean official said Jang was likely alive and in no immediate physical danger, as was his wife, Kim Kyong-hui.

Experts say Jang's removal will help the younger Kim consolidate his power base with a group of younger aides.

Jang had been a prominent fixture in many of the reports and photographs of Kim Jong-un's public activities, but his appearances have tapered off sharply this year and he has not been since in official media since early November.

Source: Agencies