Four South Koreans who were campaigning in China against human rights abuses in North Korea have been detained by Chinese security forces since their arrest in March.

The group including Kim Young-hwan, an activist for the Network for North Korean Democracy and Human Rights, was arrested in northeast China's Liaoning Province for helping defectors there, the South Korean Consulate-General in the provincial capital of Shenyang said on Monday.

They were reportedly arrested on charges of posing a threat to China's national security. The Consulate-General said all attempts to contact three of the activists have been denied, but that they managed to speak to Kim.

Korea's foreign ministry has requested Beijing deal with the case in a swift and fair manner.