North Korea Tuesday described a South Korean activist who was detained and allegedly tortured by China as a "wicked traitor" and threatened to punish him and other Seoul activists whom it named.

It accused them of involvement in alleged 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", said a statement in the name of the Committee for the Peaceful Reunification of Korea (CPRK).

In recent months the North has repeatedly threatened the South's government and also its conservative media for perceived insults to its regime.

But it is unusual for Pyongyang to single out activists, one of whom is also a legislator, by name for retribution.

In a separate statement, Pyongyang's foreign ministry vowed to further strengthen nuclear capabilities and accused the United States of attempting to topple its regime.

Pyongyang has accused Seoul and Washington of sending a spy to try to blow up statues of late leader Kim Il-Sung.

Jon Yong-Chol was presented at a news conference in Pyongyang in July and claimed he had been promised handsome rewards from Seoul intelligence agents if he succeeded in his mission.

One of the activists who was threatened Tuesday was Kim Young-Hwan, who works to help refugees who have fled the North for China.

He and three other activists were arrested on March 29 by Beijing and accused of endangering national security. They were deported on July 20.

China is North Korea's sole major ally and is generally hostile to efforts by activists such as Kim to help fugitives from the North.

In an interview with Tuesday's Chosun Ilbo newspaper, Kim said Chinese security agents repeatedly beat him, stopped him from sleeping for days and stuck an electric prod into his chest and back.

South Korea's foreign ministry said it would "actively support" any request for a United Nations inquiry into claims that Kim was tortured.

Kim is the former leader of an underground leftist party who met Kim Il-Sung in Pyongyang in 1991. He later became a fierce regime critic and now works for a Seoul-based rights group.

The CPRK, which is in charge of cross-border affairs, told the United States and South Korea to stop "luring and abducting" its citizens for "state-sponsored terrorism" and to disband groups plotting against Pyongyang.

Otherwise, it said, it would punish individuals such as Kim Young-Hwan.

It also named Kim Song-Min, described as a representative of "Radio Free North Korea" and "prime mover of the recent attempted terrorism".

Also named were Park Sang-Hak, described as a representative of the "Federation of the Movement for Free North Korea"; and Cho Myung-Chul, former director of the South's Education Centre for Unification.

Park is a leading figure in sending anti-Pyongyang leaflets across the border. Former defector Cho became a South Korean ruling party legislator in elections in April.

Hopes that the North's new leader Kim Jong-Un would set a new policy course, after decades of confrontation with the West, have not materialised.

"To expect policy change and reform and opening from the DPRK (North Korea) is nothing but a foolish and silly dream, just like wanting the sun to rise in the west," the CPRK said Sunday.