Its Envoy for Religious Freedom David Saperstein (pictured below) was due to meet the Christian human rights lawyer Zhang Kai and other activists last Wednesday, however the Chinese government arrested them before this could happen.
Zhang Kai's lawyer has said he was arrested for "undermining national security" and "disturbing social order".
He has been representing Christians in the western Chinese province of Zheijiang, which has been called the Jerusalem of the East because of the fast growth of Christianity there.
In Zheijiang and across China, authorities have been removing hundreds of church crosses, and demolishing some churches altogether.
The Chinese government has said it has removed the crosses because they are a health and safety risk.
Christian worship outside of state-sanctioned churches is illegal in China.
David Saperstein told the BBC: "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."
