Beijing has expelled dozens of South Korean missionaries working in northeast China including some helping North Korean refugees, activists said Tuesday, as relations sour over Seoul's planned US missile defence system.

Around 170 South Koreans -- including 70 missionaries and their family members -- in the province of Jilin bordering North Korea were expelled between January 10 and February 10, human rights activist Pastor Kim Hee-Tae told AFP.

"Chinese authorities raided the homes of the missionaries, citing a problem with their visas, and told them to leave," Kim said, adding that most of them were on tourist or student visas.

The number of registered South Korean missionaries in China stands at around 500. Activists say some 2,000 Christians are offering humanitarian services there, including helping North Korean fugitives in their perilous flight.

Kim said around 20 percent of the expelled missionaries were looking after North Korean refugees. He said some 40 fugitives were believed to have been repatriated to the North as a result of the raids.

North Korean fugitives typically cross first into China, then attempt to travel on via a third country to South Korea. They can face harsh punishment if sent back.

Some missionary groups in the South are voluntarily recalling workers as Chinese authorities continue their crackdown, Kim said.

South Korea's foreign ministry has advised missionary groups to take "extra caution" when working overseas.

Although China bans foreign missionary work, authorities in the past have turned a blind eye to groups operating in the impoverished northeast as they provide badly needed funds and supplies.

"Such group expulsion is unprecedented," said Jeon Ho-Jung, a spokesman for the Korea World Missions Association, adding that the crackdown appeared to be another fallout from the diplomatic tensions.

Seoul and Washington agreed last year to install the Terminal High Altitude Area Defence (THAAD) system in the South following a series of North Korean nuclear and missile tests.

The move infuriated China, which fears the system's radar will be able to spy on its own missile capabilities.

Beijing has imposed what Seoul sees as a string of retaliatory sanctions, including new restrictions on Korean celebrities performing in China and tougher customs rules for Korean products.

Last week Lotte Group -- South Korea's fifth largest conglomerate -- said it had been forced to suspend a multi-billion dollar theme park project in China after authorities raised issues with safety measures.