BEIJING (AP) ― Chinese police are holding a South Korean Bible instructor and his wife following a raid on an underground Protestant church, an activist group said Wednesday, as the government pressures Christians worshipping outside the Communist-controlled church.

The instructor, whose Chinese name was given as Jin Yongzhe, was detained Tuesday along with dozens of other Christians during a police assault on a three-floor church building in the central province of Henan’s Weishi county, the U.S.-based China Aid Association said.

The church building was searched and thousands of dollars worth of property seized during the raid, which the association said targeted a religious education seminar being held there.

After being held overnight, 49 Chinese citizens and two other South Korean pastors were released, association founder Bob Fu said. He added that he understood the South Koreans had been expelled from China, but South Korea’s Foreign Ministry said it had no information to release on the matter.

Fu said Jin and his Chinese wife, Li Sha, had likely been charged with the minor crime of illegal assembly.

South Korean church groups have long maintained close ties with Chinese believers, often operating clandestinely because of Chinese laws against proselytizing. The practice has become so common that South Korean citizens traveling to China have reported having been asked to sign pledges not to engage in missionary or other proscribed religious activity, Fu said.

Earlier in the day, an official with the Weishi county religious affairs bureau confirmed the detentions but gave no details. Like many Chinese officials, he gave only his surname, Sun. Later calls to the bureau to confirm the releases rang unanswered.

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 a strongly evangelistic character.

Crackdowns on unregistered groups appear to be increasing as intolerance of any form of dissent grows, although accurate figures are difficult to come by, Fu said. Many congregations whose activities had previously been tolerated are now coming under pressure to disband and have their members attend churches run by the official Three-Self Patriotic Movement and China Christian Council.