South Korea said Tuesday it would "actively support" any request for a United Nations inquiry into claims a Seoul activist was tortured while detained in China after helping North Korean refugees.

The foreign ministry also said it would try to interview all of some 670 South Koreans imprisoned in China to see if they suffered physical abuse.

Kim Young-Hwan and three other activists were arrested on March 29 and accused of endangering Beijing's national security. After they were deported on July 20, Kim said he had been physically abused.

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

"I will launch civil suits against China's central government or state security agency, or the security department of Dandong," Kim was quoted as saying, referring to the city bordering North Korea where he was detained.

The activist said Chinese agents had threatened to send him to North Korea while torturing him with electric shocks.

Kim's supporters have vowed to take the case to bodies such as the UN Commission on Human Rights if Beijing fails to apologise.

Foreign ministry spokesman Cho Tai-Young again urged Beijing to launch a thorough investigation, saying Seoul had received no response so far.

"If Kim addresses the issue via the UN or other international bodies... we will actively support the effort," Cho said.

"We will also probe any abuses of our nationals jailed in China... and will take necessary actions depending on the result of interviews with all of them," he said without elaborating.

The ministry has been under fire for its perceived weak response to the torture claims, which have sparked widespread anger.

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 activists such as Kim to help the fugitives.

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.