South Korea security summit over North execution
South Korea's president has convened a meeting of security officials after the shock execution of North Korean leader Kim Jong-un's uncle.
Ahead of the meeting President Park Geun-hye warned of possible "reckless provocations" by the North and called for increased border vigilance.
Last week's execution of Chang Song-thaek left the region in a "grave and unpredictable" situation, she said.
Mr Chang, a key figure, was executed for allegedly planning a coup.
The move - together with the recall of a North Korean business team from China - prompted concerns that Mr Chang's associates were being purged as part of a campaign by Kim Jong-un to consolidate his power.
Several North Koreans - calling clandestinely to friends in the South - have reported that indoctrination sessions have increased and that people are being required to write letters of loyalty in support of the country's leader.
There's an overall "state of fear" said one defector with several contacts inside the North. At least one organisation operating undercover there has suspended all projects in the wake of the execution. People there are "surprised and baffled" about what's going on, an employee said.
But others say that North Korean residents are sceptical of the purge. One trader told a contact here in Seoul that "half the public in North Korea believe Chang Song-thaek was a scapegoat - purged to take the blame for the country's economic failures".
He also said that, away from the public indoctrination sessions, North Koreans were discreetly talking amongst themselves, asking how Kim Jong-un could do this to his uncle?
China's foreign minister, meanwhile, said he believed an "important change" was taking place inside North Korea.
China - which in state media has called for Kim Jong-un to visit Beijing - was "closely watching" the situation, Wang Yi said.'Business as usual'
"Given the latest development in the North, it is uncertain in what direction its political situation would evolve," Ms Park said early on Monday.
"We also can't rule out the possibility of contingencies such as reckless provocations," she added.
The president later met her foreign affairs and security officials in a specially convened session to discuss events in the North.
Meanwhile, North Korea has been keen to present an image of business as usual, the BBC's Lucy Williamson in Seoul reports.
A government official in Pyongyang said on Sunday that the country's economic plans would carry on undisrupted.
At the same time, Mr Kim has continued with his usual public appearances, visiting a military design institute and a construction site.
The young leader's uncle was executed on Thursday after appearing before a special tribunal, state media said.
- 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"
He was accused of multiple crimes, including damaging the economy and trying to build a power-base.
Mr Chang had been seen as a power-behind-the-throne figure and a mentor to Kim Jong-un.
His execution has sparked fears of military action by Pyongyang as a way to build unity in the country, our correspondent adds.
On Sunday, US Secretary of State John Kerry called the execution an "ominous sign" of instability inside North Korea.
"It tells us a lot about, first of all, how ruthless and reckless he is," Mr Kerry said of Kim Jong-un. "And it also tells us a lot about how insecure he is, to a certain degree.
"The insights that we have tell us that he is spontaneous, erratic, still worried about his place in the power structure, and manoeuvring to eliminate any potential kind of adversary or competitor, and does so obviously ruthlessly."
In a separate development, Mr Chang's wife - Kim Jong-un's aunt - was named to a state committee on Sunday, indicating that she remained in a position of influence.
Kim Kyung-hee is the younger sister of late leader Kim Jong-il.North Korea's secretive 'first family'
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''.