The attacks would be revenge for recent high-profile defections to the South
South Korea warned on Sunday of possible North Korean assassinations and kidnappings in revenge for recent high-profile defections to the South.
With tensions also high before a large-scale South Korea-US military exercise starting from Monday, the Unification Ministry in Seoul said Pyongyang was bent on provocation.
A ministry official told reporters the defection to Seoul of North Korea's deputy ambassador to Britain and his family had put the North in "a very difficult situation".
"Considering (North Korean leader) Kim Jong-Un's character, it is very dangerous," the official said on condition of anonymity.
"It's highly likely that North Korea will make various attempts to prevent further defections and unrest among its people."
The official cited assassination attempts and terror attacks against defectors in the South as well as the abduction of South Koreans abroad as possible provocations.
He noted attempts to assassinate Hwang Jang-Yop, the North's chief ideologue and former tutor to previous leader Kim Jong-Il, who defected to the South in 1997 and died of natural causes in 2010.
The official also said Kim Jong-Un had dispatched squads to the Chinese border "to harm South Koreans" following the defection in April of a group of North Korean overseas restaurant workers.
Deputy ambassador Thae Yong-Ho was believed to have worked at the embassy in London for 10 years before his defection announced last week.
The South said he was driven by disgust for the Pyongyang regime, admiration for South Korea's free and democratic system and concerns for his family's future.
North Korea, in a vitriolic response Saturday, claimed that the "human scum" had embezzled state funds, raped a minor and spied for the South and had fled "for fear of legal punishment for his crimes".
The 12 waitresses and their manager had been working at a North Korea-themed restaurant in China. They made headlines when they arrived in the South in April as the largest group defection for years.
While Seoul said they fled voluntarily, Pyongyang claimed they were kidnapped by South Korea’s National Intelligence Service and waged a vocal campaign through its state media -- including interviews with family members -- for their return.
Seoul said last week the 13-strong group was now free to settle in South Korean society after the intelligence service completed investigations into their case and allowed them to leave custody.
A spokesman for the North's emergency committee set up for "rescuing" abductees described that announcement as a "mean plot" aimed at "covering up the truth behind the group abduction".
"Keeping them hidden from the public... citing 'safety reasons' shows that the puppet government's announcement is a complete fabrication," he said.
"We will continue fighting until we can rescue and bring back our female citizens."
Nearly 30,000 North Koreans have fled poverty and repression at home to settle in the capitalist South.
But group defections are rare, especially by staff who work in the North Korea-themed restaurants overseas and are handpicked from families considered "loyal" to the regime.
On Monday Seoul and Washington begin their annual joint Ulchi Freedom Guardian (UFG) exercise involving tens of thousands of troops.
"With the start of the UFG drills tomorrow, we ask all citizens to be on guard for North Korean terror threats and cooperate with the government's efforts," said the unification ministry official.
The two-week drill is one of a number of annual joint exercises viewed as provocative rehearsals for invasion by Pyongyang.
On Sunday a spokesman for North Korea's Committee for the Peaceful Reunification of Korea said Pyongyang's determination to retaliate "will only grow stronger" if such exercises continue.
"It is our firm position that we will mercilessly attack the threats of invasion and provocation from the enemies with our nuclear deterrence," the spokesman said in a statement on the North's official KCNA news agency.