Beijing, China - China recently detained an underground Roman Catholic priest and 10 seminarians, a rights group said in a statement seen by Reuters on Friday.
Word of last Saturday's detentions came just ahead of a planned visit to Beijing by U.S. President George W. Bush during which he is expected to press China on religious freedom and attend a church service.
Beijing has had no ties with the Vatican since 1951 and insists relations cannot be resumed unless the Holy See severs links with self-ruled Taiwan, which China claims as a breakaway province.
Six of the detained seminarians from a village outside Baoding in the northern province of Hebei were released after three days in police custody, the U.S.-based Cardinal Kung Foundation said.
But the other four and Father Yang Jianwei, 33, were still believed held and their whereabouts were unknown, the foundation statement said.
Beijing says Chinese Catholics must belong to a state-backed church that does not recognize the Pope's authority.
Last month, China unusually gave its approval to a funeral for a Rome-appointed underground bishop, a move seen as a possible sign of warming ties between Beijing and the Vatican.
But recent weeks had seen a renewed campaign by the Chinese government to get members of underground churches to register with the official church and "to institute more control over underground priests by means of arbitrary detentions", the statement quoted foundation president Joseph Kung as saying.
The group had previously reported that underground bishop Jia Zhiguo, 70, was arrested early this month in Hebei for the second time this year and the eighth time since January 2004.
Last week, China sentenced a Protestant minister to three years in jail for privately publishing the Bible and other religious publications.
Bush, who is due in Beijing on Saturday, urged China this week to move further to allow religious and political freedoms in a speech delivered in Japan.
The international group Human Rights Watch issued an open letter to Bush ahead of his Beijing visit urging him to take up China's record on religion and other rights when he meets President Hu Jintao and other leaders.
A former Vatican foreign minister is to visit Taiwan and meet President Chen Shui-bian next week amid speculation that the Vatican may break ties with the island and open an embassy in China.
Last month, Pope Benedict's top diplomat said the Holy See was always ready to end relations with Taiwan and return its embassy to Beijing but China must respect religious freedom and treat the Vatican fairly.