BEIJING, May 8 (Reuter) - A U.S.-based Tibetan human rights group has denounced China for jailing a monk convicted of colluding with the Dalai Lama during a search for the reincarnation of the Himalayan region's second holiest cleric.

A court in Tibet's Xigaze prefecture last month sentenced Chadrel Rinpoche, the former head of the committee that ran the Tashilunpo lamasery, to six years in prison for trying to split the country and leaking state secrets.

The monk and former official had colluded with Tibet's exiled spiritual leader, the Dalai Lama, while serving as the head of a team searching for the reincarnated "soul boy" of the 10th Panchen Lama, who died in 1989, a court official said.

"(Chadrel Rinpoche) only followed customary Tibetan religious tradition in his efforts to find the true reincarnation of the 10th Panchen Lama," said Lodi Gyari of the Washington-based International Campaign For Tibet.

"This blatant action is a direct attack on the core religious belief of the Tibetan people, and will further alienate the Tibetan people from the Chinese rulers," he said in a statement.

Local Chinese-language media made no mention of Chadrel Rinpoche's jailing in a news blackout apparently imposed out of fear of triggering anti-Chinese protests.

Chadrel Rinpoche was accused of helping the Dalai Lama to choose a 6-year-old Tibetan boy, Gedhun Choekyi Nyima, as the new Panchen Lama in May, 1995.

The move enraged atheist China which named another 6-year-old Tibetan, Gyaincain Norbu, instead.

Many Tibetans see the Beijing-anointed boy as a false pretender. The whereabouts of Gedhun Choekyi Nyima are not known.

Co-defendants Qamba Qung, a former government official, and Samzhub, a businessman, were sentenced to four and two years in prison respectively.

The International Campaign for Tibet said Qamba Qung was also a monk and an assistant to Chadrel Rinpoche.

The sentences were handed down on April 21 and went into effect on May 5 after the three defendants "indicated acceptance of the rulings and would not appeal," the official Xinhua news agency said Wednesday.

China has come under fire from rights groups and Western governments for its treatment of those seeking independence for Tibet.

Beijing dismisses criticism as interference in its internal affairs and has slammed the West for using Tibet as a tool to split China.

Chinese troops marched into Tibet in 1951 and suppressed a rebellion in the region in 1959 that forced the Dalai Lama and thousands of followers to flee into exile in India.

The Dalai Lama won the Nobel Peace Prize in 1989 for his non-violent campaign for more autonomy for his homeland.

Tibet has been rocked in recent years by sporadic, sometimes violent, anti-Chinese unrest, with monks and nuns often at the forefront of demonstrations for independence.

Authorities tightened security and launched a manhunt across the deeply religious region, which borders India, Nepal, Bhutan and Burma, after a bomb exploded on Christmas Day outside government offices in Tibet's capital, Lhasa.