The once-powerful uncle of North Korean leader Kim Jong Un has been executed for a string of alleged crimes, CNN reported.

State news agency KCNA announced his death early on Friday, Dec 13 branding the once-powerful Jang Song-Thaek a "traitor".

Jang was executed on Thursday shortly after a special military trial, the agency reported, after committing such a "hideous crime as attempting to overthrow the state by all sorts of intrigues and despicable methods with a wild ambition to grab the supreme power of our party and state".

South Korea media say they believe he was killed by machine gunfire - a relatively common form of execution in the North.

The announcement comes days after Pyongyang announced that Jang had been removed from all his posts because of allegations of corruption, drug use, gambling, womanising and generally leading a "dissolute and depraved life".

He was once considered the second most powerful official in the North. He was seen as helping Kim Jong Un consolidate power after the death of his father, Kim Jong Il, two years ago.

Jang is the latest and most significant in a series of personnel reshuffles that Kim has conducted in an apparent effort to bolster his power.

State TV this week showed photos of Jang being dragged out of his seat at a meeting by two officers, in an extremely rare public humiliation of a figure who was then demonised as a drug-taking womaniser.

South Korean President Park Geun-Hye on Tuesday accused Kim Jong Un of resorting to extreme violence to cement his leadership.

"North Korea is now engaged in a reign of terror while carrying out a massive purge to consolidate the power of Kim Jong Un," she told a cabinet meeting, according to her office. The Kim family has ruled the North for six decades with an iron fist, regularly purging those showing the slightest sign of dissent. Most are executed or sent to prison camps.