Four priests from China's underground Roman Catholic Church have been detained by police, a U.S.-based monitoring group said Sunday.

Three priests were detained Tuesday in the northern region of Inner Mongolia after fleeing their hometown to avoid arrest for refusing to join the state-sanctioned church, the Cardinal Kung Foundation announced. It said the fourth priest was detained in early July in the northern province of Hebei following a motorcycle accident.

It gave no details of what charges the priests might face.

China's Catholics are permitted to worship only in churches run by a government-monitored group with no ties to the Vatican. But millions who remain loyal to the pope worship in secret "house churches."

The priests detained in Inner Mongolia's Xilin Gol League region were identified as Liang Aijun, 35, Wang Zhong, 41, and Gao Jinbao, 34. All were from Hebei, according to the Kung Foundation, which is headquartered in Stamford, Connecticut.

Officers who answered the phone at police headquarters in the cities of Xilinhot and Erlianhot in Xilin Gol said they had no information on the cases. They all refused to give their names.

The fourth priest was Cui Tai, 50, of Hebei's Zhuolu county, the group said.

The Kung Foundation says five bishops and 15 priests or lay people from the underground Catholic church are in jail, while others are under house arrest or police surveillance.

"We urge the Chinese government to take steps immediately to stop all persecution throughout China and release all Roman Catholic bishops and clergy together with those faithful of other faith from prisons," the group's president, Joseph Kung, said in the statement.

The group is named for the late Cardinal Ignatius Kung Pinmei of Shanghai, who spent 30 years in Chinese prisons and died in the United States in 2000 at age 98.