The US has called for the release of 30 leaders of one of China's largest underground church network who were reportedly detained over the weekend in overnight raids in various cities.

The list includes several pastors and Zion Church founder Jin Mingri who was arrested in the early hours of Saturday after 10 officers searched his home, US-based non-profit ChinaAid said.

The Chinese Communist Party promotes atheism and tightly controls religion - still, some Christian groups are calling this the most extensive crackdown against the faith in decades.

Christians have long been pressured to join only state-sanctioned churches that are led by government-approved pastors and toe the party line.

It is unclear if the detainees have been formally charged.

"Such systematic persecution is not only an affront to the Church of God but also a public challenge to the international community," Zion Church said in a statement.

Urging China to release the church leaders, US Secretary of State Marco Rubio said in a statement on Sunday that "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".

Former US vice-president Mike Pence and former secretary of state Mike Pompeo have also released statements on X condemning the arrests.

When asked about the arrests at a press conference, Chinese foreign ministry spokesperson Lin Jian said he was not aware of the case.

He added: "The Chinese government governs religious affairs in accordance with the law, and protects the religious freedom of citizens and normal religious activities. We firmly oppose the US interfering in China's internal affairs with so-called religious issues."

This could be yet another source of friction in the US-China relationship with trade tensions once again ramping up between the world's two biggest economies over tariffs and export controls.

Already, there is doubt over whether a summit between US President Donald Trump and his Chinese counterpart Xi Jinping, which was expected to happen in South Korea later this month, will proceed.

Under Xi, Beijing has cracked down even more on religious freedom, especially against Christians and Muslims.

At a national conference on religion in 2016, he called on the party to "guide those [who are] religious to love their country, protect the unification of their motherland and serve the overall interests of the Chinese nation".

Despite this, there has been a growing movement of unregistered house churches in China.

Among them is Zion Church, which Mr Jin started in 2007 with just 20 people. Its network now includes some 10,000 people in 40 cities across the country, making it one of the largest underground churches in China.

In September 2018, the Party officially banned the church after it resisted government pressure to install security cameras at its property in Beijing. Mr Jin and several church leaders were detained briefly.

Many of its branch congregations across the country have since been investigated and shut down. Mr Jin's family relocated to the US for safety, while he remained in China to pastor his flock. Authorities have barred him from leaving the country.

Still the church continued to gather in small groups and shared its sermons online.

ChinaAid has called this roundup of Christian leaders - which involved police across several cities - unprecedented, and the "most extensive and coordinated wave of persecution" against Christians in over four decades.

"This new nationwide campaign echoes the darkest days of the 1980s, when urban churches first re-emerged from the Cultural Revolution," said ChinaAid's founder Bob Fu, referring to a period of mass purges in the 1960s and 1970s which triggered violence and huge upheaval across China.

In a letter seeking prayers, Mr Jin's wife Liu Chunli wrote that her heart is "filled with a mix of shock, grief, sorrow, worry, and righteous anger".

Mr Jin "simply [did] what any faithful pastor would do... He is innocent!" she wrote, adding that her family's hopes for a reunion after being separated for more than seven years have been dashed yet again.

Several house churches in China have also issued statements calling for the release of those detained.

Sean Long, a Zion Church pastor based in the US, said Mr Jin had been prepared for a crackdown of this scale.

In a Zoom call weeks ago between the two pastors, Mr Long had asked what would happen if Mr Jin was put in prison and all the church's leaders detained.

Mr Jin had replied: "Hallelujah! For a new wave of revival will follow then!"