Four leaders in China's underground Protestant church were sentenced to two-year terms in a labor camp, a church monitoring group said Friday.

The four were sentenced to "reeducation through labor" on Tuesday, nearly one month after they were detained at a police station in Langzhong, a city in the southwestern province of Sichuan, according to the U.S.-based China Aid Association.

The association identified the four as Li Ming, Wang Yuan, Li Mingbo, and Jin Jirong, saying they were pastors and leaders in the Chinese House Church Alliance, which groups nongovernment-sanctioned churches from across the country.

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It said they were seized while appealing to officers for the release of 14 congregants seized in a raid on a non-sanctioned church service that morning.

Chinese law permits police to pass sentences of up to three years in labor camps without trial, a system ostensibly intended to convert minor criminals into upstanding citizens.

Critics say the system is frequently used to silence religious, political and ethnic dissidents.

China allows worship only in churches registered with the Communist Party-controlled official church association, and has cracked down hard on unofficial groups.

Despite that, millions of Chinese still risk harassment and arrest by worshipping independently, often in private homes.

An officer who answered the phone at Langzhong police headquarters said he wasn't authorized to release any information and refused to give his name.