South Korea plans to demand an apology and other diplomatic steps from China after verifying a claim by a prominent rights activist of abuse during his detention, Seoul officials said Thursday.

Kim Young-hwan and three other South Korean crusaders for human rights in North Korea were deported to Seoul last week after 114 days of confinement in China. They were arrested on March 29 on charges of “endangering national security.”

Kim said Wednesday that he was tortured, deprived of sleep and forced into 13 hours of daily labor by Chinese investigators during the period and told not to disclose the abuse.

“If his claim proves to be true, we will demand that China apologize and take stern action to prevent such an incident from occurring again,” deputy Foreign Ministry spokesperson Han Hye-jin said at a media briefing.

The ministry said Wednesday it lodged a complaint with China after Kim told Korean consuls on June 11 about the abuse.

The ministry summoned Chinese Ambassador to Seoul Zhang Xinsen shortly after the meeting. On Monday, it called in Zhang’s proxy Chen Hai to ask for a “strict, thorough reinvestigation,” Han said.

Beijing has denied the claim despite Seoul’s six rounds of verification requests.

However, Han indicated difficulties due to a lack of medical evidence that could help cement Kim’s claim.

Kim, 49, was a pro-North Korea movement leader playing a key role in disseminating the North’s s philosophy of “juche,” or self-reliance.

He transitioned to activism against the iron-fisted regime’s human rights abuses in the 1990s and is currently a senior researcher for the Seoul-based Network for North Korean Democracy and Human Rights.

By Shin Hyon-hee  (heeshin@heraldcorp.com)