North Korea said Friday that it had executed Kim Jong-un's uncle as a traitor for trying to seize supreme power, a stunning end for the leader's former mentor, long considered the country's No. 2.

In a sharp reversal of the popular image of Jang Song Thaek as a kindly uncle guiding leader Kim as he consolidated power, the North's official Korean Central News Agency indicated that Jang instead saw the death of Kim's father, Kim Jong-il, in December 2011 as an opportunity to challenge his nephew and win power.

Jang had been tried and executed, North Korea said, for "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." It called him a "traitor to the nation for all ages" and "worse than a dog."

Some analysts see the public pillorying of such a senior official, and one related to the leader, as a sign of Kim coming into his own. But others see signs of dangerous instability and a rare acknowledgement that behind the scenes, Kim's rise has not been as smooth as previously thought.

Fears of instability after execution

The unusually detailed announcement came only days after North Korea said it had "eliminated" Jang from all his posts. Despite the strong language and allegations in the announcement on Monday of Jang's fall, there had been no sign in North Korean media of an imminent execution.

North Korea Kim Uncle

North Korean leader Kim Jong-un's uncle Jang Song Thaek has been executed by the regime. The fate of his wife remains unclear. (Kyodo News/Associated Press)

Kim has overseen other high-profile purges since taking over after the death of his father, Kim Jong-il, two years ago. But none of the purges have been as public — or as close to home — as the downfall of Jang.

Analysts say Kim Jong-un has acted swiftly and ruthlessly to bolster his own power and show strength, but there are fears in Seoul that the removal of Jang and his followers could lead to instability, a miscalculation or even attack on the South.

Jang had been seen by outsiders as the leading supporter of Chinese-style economic reforms and an important link between Pyongyang and Beijing.

In Seoul, top presidential security and government ministers began an unscheduled meeting Friday to discuss Jang's execution and its aftermath, according to the presidential Blue House.

'Some ripple effect' expected

During his two years in power, Kim has overseen nuclear and missile tests, other high-profile purges and a barrage of threats this spring, including vows of nuclear strikes against Washington and Seoul. His father, Kim Jong-il, took a much lower public profile when he rose to power after the death of his father, Kim Il-sung, in 1994.

Although the high-level purges could indicate confidence, Victor Cha, a former senior White House adviser on Asia, said he sees signs of "a lot of churn in the system."

"If he has to go as high as purging and then executing Jang, it tells you that everything's not normal in the system," said Cha, an analyst at the Center for Strategic and International Studies think tank in Washington. "When you take out Jang, you're not taking out just one person — you're taking out scores if not hundreds of other people in the system. It's got to have some ripple effect."

North Korea has recently turned to attempts at diplomacy with South Korea and the United States. But tensions have remained high since Pyongyang's threats in March and April. Those included warnings that it would restart nuclear bomb fuel production.

There was no immediate word about the fate of Jang's wife, Kim Kyong Hui, the younger sister of Kim Jong-il, although some analysts believe that because she is directly related to the nation's founder and has been reportedly ill, she may be spared Jang's fate. She was also seen as an important mentor to Kim Jong-un after her brother's 2011 death.

The White House said it could not independently confirm reports of Jang's execution, but has "no reason to doubt" the report from KCNA.

Patrick Ventrell, a National Security Council spokesman, said, "if confirmed, this is another example of the extreme brutality of the North Korean regime."

Many public allegations against Jang

The KCNA report called Jang a "despicable political careerist and trickster" and "despicable human scum."

It was also unusually specific. For instance, it criticized Jang for not rising and applauding his nephew's previous appointment to a senior position because Jang "thought that if Kim Jong-un's base and system for leading the army were consolidated, this would lay a stumbling block in the way of grabbing the power."

'If confirmed, this is another example of the extreme brutality of the North Korean regime.'- Patrick Ventrell, a National Security Council spokesman

Jang was described earlier this week by state media as "abusing his power," being "engrossed in irregularities and corruption," and taking drugs and squandering money at casinos while undergoing medical treatment in a foreign country.

Some analysts were taken aback by the speed of Jang's fall.

"We didn't expect that Kim Jong-un would eliminate Jang Song Thaek so quickly," said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University.

Kim wants to increase his hold on power to the level enjoyed by his late father, but Jang's execution shows Kim's that hasn't yet happened, Lim said.

"North Korea's announcement (about Jang) is like an acknowledgement that Kim Jong-un's government is still in a transitional period," he said.

A "reign of terror," including more purges, is now likely, but Kim Jong-un will eventually ease up in his approach to domestic affairs because he'll face a bigger crisis if he fails to revive the struggling economy and improve people's living standards, Lim said.