China appears to be intensifying its crackdown on foreign Christian missionaries, arresting and detaining two more South Korean pastors for allegedly attempting to smuggle North Korean defectors out of China.

"The arrested pastors openly stated to Chinese authorities they were helping North Korean defectors out of fear they would be subject to inhumane treatment if repatriated to the North," South Korean pastor and activist Peter Jung told Yonhap news agency on Wednesday.

Chinese government agents arrested one of the pastors with his wife while about to board a plane from China to South Korea. The other pastor and his wife were arrested by Chinese agents in their hotel room. Officials later released the wives of the two pastors, who were then transferred to the Chinese city of Benxi in Liaoning Province, where they will be detained while awaiting a final decision on their cases, according to CBN News.

In the last few months, China has arrested and later deported about 30 to 70 missionaries, according to reports by Agence France-Presse (AFP).

One source told the Financial Times that even though Chinese authorities have long been targeting foreign missionaries, the recent arrests, detentions and expulsions suggest that the government is intensifying its crackdown on these missionaries.

"The missionaries were keeping a low profile. In the past, most missionaries were given a month to leave since their activities in China were not harming the country. This time, it was different," the source said.

A previous report said the recent wave of arrests and expulsions came after a series of police raids on churches.

Four other South Korean missionaries who were arrested last month were reportedly conducting evangelisation activities in China's northeast Yanji region while at the same time helping North Korean defectors in navigating the risky journey across the Yalu River, which separates China and North Korea.

Bob Fu, founder and president of China Aid, an organisation that documents persecution of Christians in China, said the reason behind China's intensified crackdown on missionaries is because "the top leadership is increasingly worried about the rapid growth of Christian faith and their public presence, and their social influence."

"It is a political fear for the Communist Party, as the number of Christians in the country far outnumber the members of the Party," Fu told The Christian Post.

This year, the non-profit group Open Doors has placed China among the worst persecutors of Christians in the world, ranking 39th in its 2017 "Watch List" of top offenders.