North Korean purge of leader's uncle sparks stability fears
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.
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.
"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.
But in early December, it emerged he had been removed from his senior military position and that two of his aides had been executed.
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.
Kim Jong-il (d)×
Kim Jong-il was one of the most secretive leaders in the world.Tales from dissidents and past aides created an image of an irrational, power-hungry man who allowed his people to starve while he enjoyed dancing girls and cognac.
But a different picture was painted by Sung Hae-rim, the sister of one of his former partners in her memoir, The Wisteria House.
She describes a devoted father and a sensitive, charismatic individual, although she admits even those closest to him were fearful of him.
North Korean media depicted him as a national hero, whose birth to the country's founder, Kim Il-sung, was marked by a double rainbow and a bright star.
The youngest sister of the late Kim Jong-il and the wife of the man formerly regarded as the second most powerful figure in North Korea, Chang Song-thaek.
She has held a wide range of important Workers' Party positions including being a member of the all-powerful Central Committee.
Her promotion to four-star general made Kim Kyung-hee the first North Korean woman ever to achieve such status.
Analysts say Kim Kyung-hee and her husband were seen as mentors for the new leader Kim Jong-un when he came to power in 2011. But news of her husband's execution in December 2013 suggests the most significant upheaval in North Korea's leadership since Mr Kim succeeded his father.
Chang Song-thaek (d)×
Chang Song-thaek was married to Kim Kyung-hee, the younger sister of the late Kim Jong-il. When the inexperienced Kim Jong-un became the new leader in 2011, the couple were widely thought to be acting as his mentors.
In December 2013, the powerful uncle - who sat on the country's top military body - was denounced by the state-run news agency for corruption. Images were shown of him being removed from a Politburo meeting by uniformed guards. He was then executed.
Mr Chang's execution is the biggest upheaval in North Korea's leadership since Mr Kim succeeded his father.
Kim Jong-nam, 39, is Kim Jong-il's eldest son.
Sung Hae-rang, the sister of Kim Jong-nam's deceased mother Sung Hae-rim, has written in her memoir that Kim Jong-il was extremely fond of Kim Jong-nam and was pained to be away from him. Like his half-brothers, Kim Jong-nam studied at an international school in Switzerland.
His chances of succession appeared to be ruined when, in 2001, Japanese officials caught him trying to sneak into Japan using a false passport. He told officials that he was planning to visit Tokyo Disneyland.
Some analysts argued that he may have been forgiven by his father, as there is precedent for the regime reinstating disgraced figures after a period of atonement. Confucian tradition also favours the oldest son.
But in a rare interview while on a trip to China last year, Kim Jong-nam said he had "no interest" in succeeding his father.
Kim Sul-song, 36, is Kim Jong-il's daughter born to his first wife, Kim Young-sook.
Reports say she has worked in the country's propaganda department, with responsibility for literary affairs.
One South Korean report said she had also served as her father's secretary.
Kim Jong-chul, 29, studied at an international school in Switzerland. He works in the WKP propaganda department.
His mother, Ko Yong-hui, is said to have been the North Korean leader's favourite consort.
However, Kenji Fujimoto, the pseudonym of a Japanese sushi chef who spent 13 years cooking for Kim Jong-il, has written that the leader considered his second son "no good because he is like a little girl".
Kim Jong-un, the second son of Kim Jong-il and his late wife Ko Yong-hui, was anointed "the great successor" by Pyongyang.
Like his older brothers, he is thought to have been educated abroad.
A Japanese sushi chef who worked for Kim Jong-il for 13 years up to 2001 said that he "resembled his father in every way, including his physical frame".
Speculation that he was being groomed to succeed his father had been rife for years.
Since taking power, he has presided over a long-range missile test, North Korea's third nuclear test and most recently the execution of his uncle, Chang Song-thaek.
Ri Sol-ju was introduced as Kim Jong-un's wife in state media reports about the opening of an amusement park in July 2012.
Reports simply said he attended the event with his wife, "Comrade Ri Sol-ju".
Little more is known about Ri Sol-Ju, although there has been much speculation about her background since pictures first emerged of Kim Jong-un with an unidentified woman. There is a North Korean singer of the same name, but she is not now thought to be the same person.
State media did not mention when the couple got married.
The grandson of Kim Jong-il and nephew of leader Kim Jong-un has said he wants to "make things better" for the people of his country.
Kim Han-sol, 17, spoke of his dreams of reunification of the two Koreas in an television interview in Bosnia, where he is studying. Kim Han-sol said he had never met his grandfather or uncle.
He described an isolated childhood spent mostly in Macau and China, after his birth in Pyongyang in 1995. In the future, he said he pictured himself going to university and then ''volunteering somewhere''.