Christian pastors are being rounded up and jailed by Chinese police in a major crackdown campaign.

Just this weekend, Chinese police arrested dozens of pastors in an "underground church" - the single largest spate of detentions in years.

One, pastor Jin Mingri, the founder of Zion Church, an unofficial "house church" not sanctioned by the Chinese Communist Party, was among those taken in.

Church spokesman Sean Long has now issued a dire warning over persecution as a result.

"What just happened is part of a new wave of religious persecution this year," Mr Long said, adding that police had questioned more than 150 worshippers and stepped up harassment at in-person Sunday services in recent months.

He told Reuters that around the same time, authorities had hauled in almost 30 pastors and church members nationwide, but later released five.

Some 20 pastors and preachers are still behind bars.

Mr Jin, 56, is being held on suspicion of "illegal use of information networks", which could see him jailed for up to seven years.

Chinese police arrested dozens of pastors in an 'underground church' this weekend alone

"He's been hospitalised in the past for diabetes. We're worried since he requires medication," his daughter Grace Jin said.

"I've also been notified that lawyers are not allowed to meet the pastors, so that is very concerning to us."

The crackdown has already seen the condemnation of US Secretary of State Marco Rubio, who called on the pastors' immediate release.

"The United States condemns the Chinese Communist Party's recent detention of dozens of leaders of the unregistered house Zion Church in China, including prominent pastor Mingri 'Ezra' Jin," he said.

Jin Mingri, the founder of Zion Churchm, was among those hauled in by CCP goons

"This crackdown further demonstrates how the CCP exercises hostility towards Christians who reject party interference in their faith and choose to worship at unregistered house churches.

"We call on the CCP to immediately release the detained church leaders and to allow all people of faith, including members of house churches, to engage in religious activities without fear of retribution."

China's state "religion regulator" last month banned unauthorised online preaching or religious training by clergymen, as well as so-called "foreign collusion".

US Secretary of State Marco Rubio has urged China to let its Christians go

Last month, President Xi Jinping pledged to "implement strict law enforcement" and to make religion in China more Chinese.

Some 44 million Christians are registered with state-sanctioned churches in China - the majority Protestant.

However, tens of millions more are said to be part of illegal "house churches" which operate outside Communist Party control.