North Korea announced Friday that it had executed the uncle of leader Kim Jong Un, calling him a “traitor for all ages” who opposed Kim’s rule and plotted his overthrow.

Jang Song Thaek was executed after admitting to his crimes before a military tribunal Thursday, the North said in a statement released by the state-run news agency. The North said Jang, 67, in attempting to realize his “wild ambition” of supreme power, was “despicable human scum” and “worse than a dog.”

Jang’s execution comes just days after he was stripped of all his positions and removed from Kim’s small inner circle of advisers. His purge, the highest-level leadership change since a power handoff two years ago, marks Kim’s boldest but riskiest step yet in eliminating potential rivals and consolidating power in his nuclear-armed police state.

Jang’s removal was documented by the North with unusual detail and suggests that Kim is willing to publicly go after adversaries in ways that his secretive father never did. Compared with his predecessors, Kim has grabbed power far more quickly — and done so in a manner more openly confrontational.

Some analysts say the execution of Jang could sufficiently terrify others in the Workers’ Party and the military and keep them in lockstep with the wishes of their leader. But the move could also backfire and raise the incentive for Kim’s top lieutenants to fight back if they ever feel they are falling out of favor.

Although North Korea has always been known for its purges, executions have been traditionally ordered only for lower-­level functionaries. Over the past few decades, high-level officials who lose favor with the Kims have been sent into the countryside and placed under house arrest, sometimes with the chance to make career comebacks, according to accounts from rights groups and defectors. The purges have almost always been done in secret.

On Friday morning, the state newspaper, the Rodong Sinmun, released photos of Jang in the courtroom, head bowed and arms cuffed. He was flanked by two security guards. A front-page article described his demise. The U.S. State Department said Jang’s execution, if confirmed, “is another example of the extreme brutality of the North Korean regime.”

“We are in absolutely uncharted territory,” said Andrei Lankov, an expert on North Korean affairs at Kookmin University in Seoul, adding: “Kim Jong Un has changed the rules of the game. He’s willing to purge the leaders closest to him, and quite publicly. Behind this I see his personal character. He seems to be far more ruthless than his father. Kim Jong Un is more inclined to kill, and he seems more impulsive and emotional.”

In recent days, the North had accused Jang of everything from graft to gambling to womanizing. But the North’s account Friday narrowed to a more serious charge, saying that Jang had contested the country’s system of passing power from father to son. In 65 years of Kim family rule, never has the North so explicitly described high-level dissent.

The North said Jang, a lifelong politician, had quietly followed marching orders under the nation’s founder, Kim Il Sung, and his son Kim Jong Il. But after Kim Jong Il’s death in December 2011, Jang made his move, “overtly and covertly standing in the way” of the succession, the statement said.

According to the North’s account, which could not be independently verified, Jang turned his own departments into personal fiefdoms. He also tried to build influence with the military, “with a foolish calculation that he would succeed in staging a coup if he mobilized the army.”

Jang was quoted in the news account as admitting, “I was going to stage the coup by using army officers who had close ties with me or by mobilizing armed forces under the control of my confidants.”

There are reasons to doubt the North’s version of events, and some experts cautioned that Kim Jong Un could be exaggerating charges as a way to rationalize the removal of a powerful figure. Until early last month, Jang remained in the public spotlight. The full account of his execution also accused him of distributing pornography among his confidants, masterminding a disastrous 2009 currency revaluation and selling off resources to China at bargain rates.

In the years before Kim Jong Un took power, Jang received a series of promotions, anointed as a de facto caretaker for the succession. He was seen, along with his wife, Kim Kyong Hui, as the adult supervisor for the “Great Successor.” Jang had hundreds of loyalists in the Workers’ Party and relatively deep connections with China. In both areas, Jang could help Kim break in.

But Kim Jong Un, in the past two years, has tried to eliminate the old guard rather than lean on it. Since Kim took power, nearly 100 of 218 party heads, government ministers and military officials have been replaced, according to the South Korean Ministry of Unification. The turnover has taken place in the party and the military, in noted contrast to the early years of Kim Jong Il’s rule, when the party remained stable and the military was reshuffled.

There have been no outward signs of instability under Kim Jong Un, and the North has largely maintained its key policies, resisting economic reform, devoting money to its weapons program and maintaining a gulag system for those accused of political crimes.

But some analysts, as well as officials in Seoul and Washington, say the North’s behavior has become more reckless. Earlier this year, Pyongyang threatened nuclear strikes against its neighbors and the United States. It temporarily cut off a lucrative industrial park project that it ran jointly with the South. It has detained U.S. citizens, including an 85-year-old Korean War veteran who was released after more than a month. All the while, Kim has devoted himself to fringe pet projects, emphasizing the building of ski and water parks while holding events for Dennis Rodman, a retired U.S. basketball player.

“I don’t see any particular guile in what he’s doing,” said Victor Cha, a former White House director of Asian affairs and the Korea chair at the Center for Strategic and International Studies in Washington. “He’s doing what dictators do, especially somebody young and inexperienced. He’s taking reckless action and doing things that are self-indulgent.”