Beijing - A court in China's Tibet Autonomous Region has sentenced eight Tibetan monks to between five years and life in prison after convicting them of bombing a local government office, the London-based Free Tibet Campaign said on Tuesday.

The group identified the eight monks from the Thangkya (Tongxia) monastery in Tibet's Gyanbe township and said they were sentenced on September 23 after a secret trial by the court in Chamdo prefecture.

Contacted by telephone, Chamdo court officials said only that they were "not clear" about the sentences and that it was "not convenient" to talk about the case.

State media reported that police arrested at least nine monks from the monastery on April 6 and that the monks had confessed to their involvement in the bombing.

Free Tibet quoted its sources as saying a total of 11 monks were arrested after the incident.

The official Xinhua news agency said the monks detonated a homemade bomb inside the township government building in the early hours of March 23 after transporting it there on a motorcycle.

The agency's report gave no details of any damage or injuries caused by the bomb, and Free Tibet said that the building targeted was "widely known to be disused and empty."

According to information given to Free Tibet, the monks were between 20 and 31 years old.

The group said two monks were sentenced to life in prison, one to 15 years, three to 10 years, one to nine years, and one to five years in prison.

It said the legal process against the monks was "shrouded in complete secrecy," with the monks denied access to lawyers and their families.

"This case, like so many others in Tibet, demonstrates the urgent need for international media and independent agencies to be allowed immediate and free access to all areas of Tibet to investigate the accounts of arbitrary detention and abuse of Tibetans that continue to emerge," Stephanie Bridgen, the director of Free Tibet, said in a statement.

Chinese police detained thousands of Tibetans earlier this year following independence protests and riots in many Tibetan areas, with virtual martial law imposed in some areas.

Pro-independence demonstrations and unrest began in the regional capital, Lhasa, and other areas on March 10, the 49th anniversary of a failed Tibetan uprising against Chinese rule.

The Chinese government said 19 people were killed in rioting from March 14 in Lhasa but the Tibetan government-in-exile said about 140 people were killed, most of them Tibetans shot by Chinese police.

Protests erupted in dozens of other Tibetan areas of China. (dpa)