A South Korean rights group said Monday it would ask the United Nations to investigate the alleged torture of a Seoul activist detained in China after helping North Korean refugees there.

Kim Young-Hwan and three other people were arrested on March 29 and accused of endangering Beijing's national security.

After the group were deported on July 20, Kim claimed he had been physically abused by Chinese security authorities. He gave no details but his colleagues said he was subjected to electric shocks.

"We will take the case to international bodies like the UN Commission on Human Rights if China continually refuses to apologise," said Choi Hong-Jae, spokesman for an organisation previously set up to appeal for Kim's release.

Kim neither opposed nor publicly supported any appeal to the UN and only expressed the desire not to create a major diplomatic dispute between Seoul and Beijing, Choi said.

The group is also considering launching a criminal or civil lawsuit against the Chinese government, Chosun Ilbo newspaper quoted Choi as saying.

South Korea's foreign ministry had no immediate comment. Foreign Minister Kim Sung-Hwan said last week he would press China to conduct a thorough investigation into the torture claim.

China is North Korea's sole major ally and repatriates those North Korean refugees whom it catches, despite protests from rights groups. It is generally hostile to efforts by South Korean activists to help the fugitives.

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