A Tibetan monk has died in a Chinese prison, a rights group and Tibet's exiled government said Monday, 13 years into a sentence for terrorism and separatism observers said was deeply flawed.

Relatives of Tenzin Delek Rinpoche, 65, were informed of his death by police in the southwestern city of Chengdu on Sunday, Students for a Free Tibet (SFT) said in a statement.

In a separate statement, Tibet's government-in-exile in India cited his cousin, Geshe Nyima, saying the cause of death remained unclear, and added that relatives were outside the prison Monday morning "urging the authorities to return Rinpoche's body but to no avail".

Meanwhile, Tibetans were gathering outside government offices in Delek's home county of Yajiang in Sichuan province, according to London-based group Free Tibet. A large amount of security forces have been deployed in response, the organisation said, with government offices working through the night on Sunday.

He was convicted in 2002 of separatism and being involved in a bombing in a public square and was initially condemned to death. The sentence was later commuted to life in prison and then to 20 years.

Delek's assistant, Lobsang Dhondup, was convicted around the same time and was executed in 2003. The two men's cases drew condemnation from the European Union and rights groups at the time.

Delek maintained his innocence throughout his sentence, the SFT statement said, adding that his family had asked police for his body so that they could perform funeral rites.

Radio Free Asia, which is funded by the US government, also reported the death, quoting a source in Tibet as saying: "Chinese police informed his relatives that he was seriously ill and when they rushed to visit him, they were told he was already dead."

RFA said that Delek was known to be in extremely poor health with a serious heart condition, adding he allegedly received no treatment.

SFT attributed his medical condition to "over 13 years of unjust imprisonment and torture", adding that family members had applied for medical parole for him last year, which had not been granted.

The leader of Tibet's exiled government in the north Indian hill town of Dharamsala expressed his grief over the death and said China's rejection of medical parole reflected its "continuing hard line policies".

"Such mistreatment will only generate more resentment among Tibetans," Lobsang Sangay said in the statement.

China, which has ruled Tibet since 1951, has been accused of trying to wipe out its Buddhist-based culture through political and religious repression and a flood of immigration by Han Chinese, the country's ethnic majority.

Tibet's spiritual leader the Dalai Lama fled Tibet after an abortive uprising in 1959 established his government-in-exile in Dharamsala.

Calls to the propaganda office of Chengdu public security bureau went unanswered.