SHENYANG, China (Yonhap) -- China has detained four South Korean human rights activists since March, South Korea's foreign ministry said Tuesday.

The South Koreans were arrested by Chinese police in the northeastern Chinese city of Dalian on March 29, the ministry said.

The ministry identified one of the detainees as Kim Young-hwan, a former key ideologue of North Korea's guiding "juche" philosophy of self-reliance.

Kim met with North Korean leader Kim Il-sung in 1991 after sneaking into the North via a North Korean submersible. However, Kim later renounced his pro-North Korean ideology and got involved in projects to raise awareness about the dismal human rights of North Koreans.

A South Korean diplomat in the northeastern Chinese city of Shenyang met with Kim on April 26, the ministry said.

South Korea asked China to handle the case in a fair and swift manner, the ministry said, though the ministry declined to confirm the charges against the four activists on the grounds that they were not indicted yet.

Meanwhile, a South Korean rights group claimed that Kim was arrested on charges of endangering China's national security, citing an unidentified South Korean consul in Shenyang.

The charges against the three other South Koreans were not immediately available as South Korean consular officials were denied access to them, according to the Network for North Korean Democracy and Human Rights.

Kim is believed to be detained in the Chinese border city of Dandong across the Yalu River from North Korea, according to the Seoul-based rights group.

China is also believed to be investigating the three other South Koreans, but did not say where they are being held, according to the rights group.

"We strongly urge China to immediately guarantee South Korea's consular access to the South Koreans forcibly being held," the rights group said in a statement.

In Beijing, Huang Zhipeng, a foreign ministry official handling South Korean affairs, said he has no information. Chinese State Security Ministry officials were not immediately available for comment.