A Tibetan monk who was one of China's most prominent political prisoners has died in jail, a relative and the Tibetan government-in-exile said on Monday, raising the prospect of an increase in resentment of the government in Tibetan regions.

The United States, the European Union and international rights groups had called for the release of Tenzin Delek Rinpoche, 65, who was serving a 20-year sentence on charges of "crimes of terror and incitement of separatism".

He died on Sunday in Chuandong prison in the southwestern city of Chengdu, his cousin, Geshe Nyima, told Reuters by telephone from India. The Tibetan government-in-exile, the Central Tibetan Administration, also confirmed his death.

"Despite all our efforts at the international level to get his release and lately for his medical treatment, China has disregarded all international pleas on his behalf," said Tashi Phuntsok, spokesman for the India-based administration.

The cause of death was not clear, the monk's cousin said, but rights groups had earlier said Tenzin Delek was suffering from a heart condition. The authorities notified his sisters about his death on Sunday, the cousin added.

Telephone calls seeking comment from officials in Chuandong prison went unanswered.

Tenzin Delek was sentenced to death with a two-year reprieve in 2002, according to Students for a Free Tibet, a rights group that has campaigned for his release. The sentence was later commuted to life imprisonment and reduced to a term of 20 years.

"He is one of the most respected leaders," said Dorjee Tseten, Asia director of Students for a Free Tibet. "His death is a shock for all of us."

Tibetan writer Tsering Woeser described Tenzin Delek as one of the world's most well-known Tibetan political prisoners in the 21st century.

Tenzin Delek was also a supporter of the Dalai Lama, Tibet's spiritual leader whom China says is seeking to use violent means to establish an independent Tibet.

The Dalai Lama, who has been living in exile in India since 1959 after an abortive uprising against Chinese rule, says he wants genuine autonomy for Tibet and denies espousing violence.

On Monday, China's foreign ministry said it "firmly opposed" the participation of White House advisor Valerie Jarrett in a birthday celebration for the Dalai Lama in New York on July 10.