Talks are still underway to secure the release of four South Korean activists held in China for helping North Korean refugees, a report said Wednesday.

Despite earlier reports that Kim Yong-Hwan and three others had already been freed and were preparing to return to Seoul, sources in Beijing quoted by South Korea's Yonhap news agency said they were still being held.

"There are talks going on with Chinese authorities towards releasing the activists," one source in Beijing said. "But they're still in detention."

A South Korean foreign ministry official quoted by Yonhap said he expected a positive conclusion, adding that the two sides were taking some final steps.

The official said Seoul had no information on the timing of the release and the four had not yet left the city of Dandong on the border with North Korea, where they have been held.

Earlier Wednesday Yonhap quoted a Seoul-based source as saying the four had been released from a Dandong detention centre and would soon be deported.

Kim and the other three were arrested on March 29 and accused of endangering China's national security, a charge that can carry severe punishment.

Almost all refugees from the North cross first to China, which repatriates any fugitives it catches, classing them as economic migrants.

Rights groups have urged Beijing to treat them as potential refugees, saying returnees can face harsh punishment.

South Korean activists engage in secret activities in China to help the refugees travel on to South Korea.