A prominent Tibetan monk serving life imprisonment in a prison in China has died, a rights group said.

Sixty-five-year-old Tenzin Delek Rinpoche passed away in a Chinese prison as confirmed by his family members who were informed by the police on Sunday, Students For A Free Tibet said in a statement.

Rinpoche, who was found guilty of setting of explosions and inciting separatism in 2002, was serving a life sentence in Chuandong prison in Chengdu city, the capital of Sichuan province. The EU, U.S. and several rights groups had slammed China over his sentence at the time and called for his release, reported BBC.

"We are very sad; it's shocking news for us. He has been a very prominent religious teacher. His case has been brought up internationally for years," said Tashi Phuntsok of Central Tibetan Administration, according to CNN.

The India-based Tibetan government-in-exile said the cause of death remained unclear and claimed that Rinpoche had been in poor health with a serious heart condition.

"It is sad to learn about the death of Tulku Tenzin Delek Rinpoche. We knew he was gravely ill and his followers made numerous appeals for a medical parole," Lobsang Sangay, who is serving as prime minister of the Tibetan government-in-exile, said in a statement.

"The fact that he was not even allowed medical parole and last wish of followers to see him reflects continuing hard line policies of the Chinese government. Such mistreatment will only generate more resentment among Tibetans," Sangay added.  

Radio Free Asia (RFA) reported that Rinpoche's sisters had traveled two weeks ago to Chengdu to visit him in prison, but Chinese authorities repeatedly blocked their attempts to see him.

"They (Rinpoche's sisters) were told that they could see him next day, or next Monday, or next Sunday, and so on. On July 12, the Chinese authorities told them that they could see Rinpoche at 11:00 a.m., but they were then informed at about 12:00 noon that he had passed away," an informed source told RFA.

The United States urged Chinese authorities to investigate circumstances surrounding his death.

"The United States had consistently urged China to release Tenzin Delek Rinpoche, most recently out of concern for his health. We urge Chinese authorities to return his body to his family or to his monastery so that customary religious rituals can be properly performed," State Department spokesperson John Kirby said in a statement.