The U.S.-based China Aid Association said all three floors of a church building in Henan province's Weishi county were searched and thousands of dollars worth of property were seized during the raid Tuesday on a religious education seminar.
Also detained were three Korean citizens, including a Bible instructor, the group said. South Korea's Foreign Ministry said it had no immediate information about the reported detentions, which reflect long-standing ties between South Korean church groups and Chinese believers.
An official with the Weishi county religious affairs bureau confirmed Wednesday the detentions but gave no details. Like many Chinese officials, he gave only his surname, Sun.
China requires all religious groups to register and accept Communist Party oversight, although millions of believers continue to worship in unregistered congregations that typically have an evangelistic, charismatic character.
That includes members of Beijing's unregistered Shouwang Church, who have recently repeatedly defied police demands and attempted to gather for banned outdoor services, leading dozens to be detained. The group, which includes numerous intellectuals, was evicted from its rented space under police pressure.
Rural congregations, drawn from poorer, less-educated communities, tend to be more vulnerable to coercion from authorities.
This month, a U.S. commission listed China as one of the worst violators of religious freedoms. Beijing denies the charge.
Tuesday's raid came on the second and final day of twice-annual high-level talks between U.S. and Chinese officials that were overshadowed by sharp U.S. criticism of China's human rights record.
In an interview published Tuesday on the website of The Atlantic magazine, U.S. Secretary of State Hillary Rodham Clinton said China's human rights record was "deplorable" and that history was not on the side of governments that resist democracy.