China must immediately release detained Christian human rights lawyers, activists and church pastors, the US has said.

US envoy for religious freedom, David Saperstein, branded the detentions of defence lawyer Zhang Kai and others a "particularly alarming development". According to AP, Saperstein had been scheduled to meet with Zhang and a number of religious leaders on August 26, but they were arrested the day before.

It emerged earlier this week that Zhang has since been sentenced to six months in so called 'black jail', along with his assistant Liu Peng, on suspicion of endangering state security and disturbing the public order. Zhang had been representing churches battling orders to remove their crosses amid a crackdown on places of worship in Wenzhou, Zhejiang province, on the east coast of China.

"There can be no excuse for the detention of these religious figures who either met with me or tried to meet with me," Saperstein told reporters.

"These detentions fit into the disturbing pattern of state intimidation of public interest lawyers, internet activists, journalists, religious leaders...They clearly underscore the precariousness of religious life in China.

"It is a source of great concern to us and we are calling on the government to immediately release all these human rights activists and religious leaders."

Texas-based human rights organisation China Aid has noted that torture is common in black jails, and inmates are refused physical, written or verbal communication with family members or legal representatives. Lawyers representing Zhang were yesterday refused a meeting with Zhejiang's provincial authorities.

Zhang has worked on behalf of more than 100 churches and in July of this year wrote and distributed a 'Cross Activists Handbook', advising church leaders on how they can use China's own constitution – which guarantees religious freedom – to defend their rights.

Estimates vary, but up to 1,700 churches are believed to have been demolished or had their crosses removed in Zhejiang in the past two years. Campaigners believe the ruling Communist party is becoming progressively more suspicious of the influence of Christianity, which is experiencing monumental growth in China.