Four South Korean activists have been detained in China since March on suspicion of spying after they interviewed North Korean refugees living in hiding there, according to an anti-Pyongyang group.
South Korea's foreign ministry confirmed the four were arrested in the northeastern Chinese city of Dalian on March 29 on charges of "endangering state security".
One of the four is Kim Young-Hwan, a former leader of an underground leftist party who became an activist opposing Pyongyang's regime. Names of the three others were not given.
A South Korean consul in the northeastern Chinese city of Shenyang met Kim on April 26.
The three others on May 3 presented through Chinese authorities written statements declining to see South Korean diplomats, the foreign ministry said.
"We're trying to confirm they have decided to give up their rights of their own free will to see consuls," a ministry spokeswoman told AFP.
South Korea has asked China to handle the case in a fair and swift manner, the ministry said. It declined to elaborate on what the four are accused of doing.
Kim Young-Hwan, 48, is a researcher for a Seoul-based group called the Network for North Korean Democracy and Human Rights.
Choi Hong-Jae, spokesman for a group set up to seek his freedom, said the four had apparently been interviewing North Korean refugees to collect information about their life in China and the situation in their homeland.
"We strongly urge China to allow them to see their relatives and guarantee them consular access," he told AFP.
Almost all refugees from the North cross first to China, which repatriates those fugitives whom it catches as economic migrants.
Rights groups have urged Beijing to treat them as potential refugees, saying returnees can face harsh punishment.
South Korean activists, many of them Christian evangelists, are engaged in secret activities in China to help the refugees travel to a third country and on to South Korea.