In China, buying or selling Christian reading materials could be enough ground to land believers in jail.

Five Christians, including a pastor, found this out recently when they were sentenced to between three to seven years in prison for purchasing and selling what authorities called "forbidden Christian devotional books," China Aid reported.

The persecution watchdog said the believers—four women and one man— belong to a Korean ethnic minority group based in China. They were arrested in June last year despite the fact that they were members of registered churches in China, China Aid said.

The five were sentenced last week by a court in the northeastern Liaoning province.

Beijing has long been cracking down on unregistered Christian churches, pastors of underground congregations, and various Christian minorities, branding them as threats to communist rule in China.

Last month, a Chinese pastor was reportedly detained and the church building searched after the congregation refused to come under state surveillance.

Jin Tianming, who leads Shouwang Church, Beijing, resisted pressure to join the Three-Self Patriotic Movement, (TSPM), the state-approved Protestant body, in 2009.

Jin is now under house arrest. Members of his church are also being harassed by Chinese authorities, according to China Aid.

Also last month, four South Korean Christian missionaries were arrested while 32 others were expelled, reports said.

Chinese police earlier conducted raids on churches as part of an ongoing crackdown against Christian missionaries.

Before they were arrested and deported, the South Korean missionaries were reportedly conducting evangelisation activities in China's northeast Yanji region while at the same time helping North Korean defectors cross the Yalu River, which separates China and North Korea.

In January, a house church pastor in southwest China's Guizhou province was tortured into "confessing" charges related to "divulging state secrets," and sentenced to two and a half years in prison, according to The Christian Post.

In its 2016 report on China, China Aid said Beijing is determined to force all religions to "surrender to the authority and leadership of the Chinese Community Party."