A prominent Tibetan environmentalist who said he was "brutally tortured" in custody has been jailed for 15 years on tomb-robbing charges that were dropped years ago, his lawyer said Friday.

Karma Samdrup, a 42-year-old art collector, went on trial this week in China's far-western region of Xinjiang on charges brought in 1998 and later dropped after he bought artefacts that turned out to have been stolen.

Prosecutors resurrected the charges and said this week that the art collector had asked the thieves to keep stealing from tombs for him for remains and coffins, his lawyer Pu Zhiqiang said.

"He says he is innocent and that Xinjiang police used false evidence and torture, and didn't provide an interpreter," Pu told AFP by phone.

"Tibetans do not touch coffins or corpses, they advocate sky burials, water burials, but not earth burials. Also, robbing graves is taboo for them," he added.

Karma Samdrup appeared in court in Yanqi county on Tuesday and was sentenced Thursday. His wife said he was physically unrecognisable, according to Pu.

"He said he had been brutally tortured, and I believe him - I last saw him in mid-January and he weighed around 90 kilogrammes (200 pounds) then. Now he's around 70 kilos, which is too thin for his height," Pu said.

In India, the Tibetan government-in-exile reacted sharply to Karma Samdrup's sentence and called on Beijing to release all "prisoners of conscience."

"Samdrup has suffered terribly and we feel that he should be given a fair trial," spokesman Thubten Samphel said by telephone from the northern Indian town of Dharamshala where the Tibeten-government-exile is headquartered.

"Our general appeal to the Chinese authorities is that all prisoners of conscience should be released immediately and if anyone is put on trial then he should get a fair and transparent trial," Samphel told AFP.

According to Human Rights Watch, Samdrup's supporters believe that his arrest and trial stem from his efforts to gain the release of his two brothers.

The brothers were arrested after attempting to highlight environmental abuses by local officials in Tibet, the New York-based rights group said.

It has expressed concern over the "increasing vulnerability of important cultural and entrepreneurial Tibetan figures to politically motivated arrests and prosecutions".