Media playback is unsupported on your device Media caption State TV in North Korea announced the execution of a "wicked political careerist"

The execution of the once-powerful uncle of North Korean leader Kim Jong-un has rekindled fears of instability in the secretive nuclear-armed state.

South Korean defence chief Kim Kawn-jin promised "heightened readiness" after the purge of Chang Song-thaek.

He said the execution could be seen as part of a "reign of terror" by the North Korean leader.

Mr Chang was executed for "acts of treachery" after appearing before a military trial.

He was dramatically removed from a special party session by armed guards earlier this week.

South Korean Unification Minister Ryoo Kihl-jae warned that the purge could be followed by military moves from Pyongyang, including another nuclear test.

Removing a single individual may not be enough to quell dissent if it is widespread Dr John Swenson-Wright, Chatham House Brutal but risky moveFive infamous purges

He told lawmakers that "the North usually curbs internal (agitation) through waging provocations externally".

North Korea carried out its third nuclear test in February, to widespread international condemnation.

As tensions rose, Pyongyang threatened attacks on Japanese, South Korean and US military targets in the region.

China, North Korea's ally and neighbour, described Mr Chang's execution as an "internal matter".

"As a neighbouring country, we hope for North Korea to maintain stability..." a foreign ministry spokesman said.

However, the BBC's Martin Patience in Beijing says the bland statement is likely to mask deep concern and will raise questions as to how much influence China actually has over the state.

Victor Cha, a former senior White House adviser on Asia, warned that Kim Jong-un's purge could spread further than Mr Chang.

"If he has to go as high as purging and then executing Chang, it tells you that everything's not normal," he said.

Image caption Commuters in Pyongyang read about the purge from public newspapers

Image caption Chang Song-thaek was often seen at Kim Jong-Un's side

"When you take out Chang, 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."

Professor Lee Jung-hoon, from South Korea's Yonsei University, told the BBC that the move showed that North Korea was "very unstable".

"[For Kim Jong-un] to go to the extent to actually purge him and execute him says a lot about the state of things in that country," he said.

In Pyongyang on Friday, residents crowded around public newspapers to read the story.

State news agency KCNA said Mr Chang had appeared before a military trial on Thursday and admitted trying to overthrow the state. He was executed immediately, it said.

A long and detailed statement described him as "despicable human scum... worse than a dog".

Mr Chang was thought to have mentored his nephew during the leadership transition from Kim Jong-il in 2011.

He was married to Kim Jong-il's sister and had held senior posts in the ruling party including vice-chairman of the powerful National Defence Commission, the North's top military body.

He was frequently pictured alongside his nephew and seen by some observers as the power behind the throne.

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But in early December, it emerged he had been removed from his senior military position and that two of his aides had been executed.

Chang Song-thaek Born 1946; marries Kim Jong-il's sister in 1972

Joins Korean Workers' Party administrative ranks in 1970s

Elected to Central Committee in 1992

Sidelined in 2004, but rehabilitated in 2006

2011: Gets top military post under Kim Jong-un

Nov 2013: Dismissed from his position

December 2013: Executed as a "traitor" Profile: Chang Song-thaek Secretive 'first family' What is known about the purge?

On Monday, KCNA broadcast footage of him being removed from a party session by uniformed guards.

The BBC's Lucy Williamson in Seoul says one theory about Mr Chang's demise is that his work with China had led him to admire some of Beijing's economic reforms.

In August 2012, Mr Chang made a high-profile trip to China where he met then-President Hu Jintao. The two sides later signed a raft of economic deals, including the development of two special economic zones.

However, our correspondent says it is more likely that Mr Chang presented a perceived threat to his nephew's authority, she says.

Analysts say the purge is bound to raise regional tensions.

Japan said it was "closely watching the situation".

"We will calmly monitor the situation while communicating with other countries and collect relevant information," Kyodo news agency quoted Chief Cabinet Secretary Yoshihide Suga as saying.

The White House said that, if confirmed, the execution was "another example of the extreme brutality of the North Korean regime".

"We are following developments in North Korea closely and consulting with our allies and partners in the region," it said in a statement.

The UK said it was "deeply concerned about the impact of this unpredictable regime on stability in the region".

"Our embassy in Pyongyang is monitoring the situation closely and we will continue to maintain close contact with our allies on this," a Foreign and Commonwealth Office spokesman said.