South Korea Wednesday rejected North Korean claims that it is planning terrorist acts as "ridiculous", and said it would protect Seoul activists threatened by Pyongyang.

Pyongyang late Tuesday said the activists were involved in US and South Korean plots to blow up statues of past leaders and stage other acts of terrorism in the North.

Offenders "will not be safe no matter where they are and they will not be able to escape merciless punishment", it said in a statement which took the unusual step of naming those singled out for possible retribution.

The United States and South Korea should stop "luring and abducting" the North's citizens for "state-sponsored terrorism" and should disband groups plotting against Pyongyang, it warned.

The North has accused the two countries of sending a spy to try to blow up statues of late leader Kim Il-Sung.

Jon Yong-Chol told a Pyongyang news conference in July he had been promised handsome rewards from Seoul intelligence agents if he succeeded.

"The North is making ridiculous accusations of kidnapping, acts of terrorism and crimes involving some North Korean defectors," said a spokeswoman for the South's unification ministry.

"None of them have any truth in them and the accusations are not worth responding to."

The ministry, which handles cross-border affairs, said it would not give details of security surrounding individual defectors.

"But about those who were named, relevant authorities like the police and the intelligence service have been taking necessary measures."

The North named Kim Young-Hwan, who was detained for more than three months and allegedly tortured by China for trying to help North Korean refugees in that country.

It also identified Kim Song-Min, a defector and head of Radio Free North Korea.

Also named were Park Sang-Hak, who fled the North and now sends anti-Pyongyang leaflets across the border; and Cho Myung-Chul, a defector who became a South Korean ruling party legislator in elections in April.

Kim Young-Hwan said he would not be cowed.

"If our operations are intimidated by such threats, that means we end up behaving the way the North intended," he told Yonhap news agency.

Cho in a statement urged South Koreans not to surrender to the North's dictatorship "and stand up and fight against it to protect the dignity and lives of South Koreans".

The North has previously tried to silence critics in the South. Last year it sent an agent posing as a defector to try to murder Park with a poison-tipped weapon.

The South's intelligence agency warned Park and arrested the agent who was later jailed for four years.

Another North Korean spy was jailed for 10 years in January 2011 for plotting to kill the highest-ranking defector ever to come south.

It was at least the second known assassination attempt on Hwang Jang-Yop, who died of natural causes at his closely guarded Seoul home in 2010.