A Tibetan monk died after self-immolating in China, state media said Monday, taking to 15 the number of people who have set themselves on fire in Tibetan areas in less than a year in apparent protest.

Nyage Sonamdrugyu, 40, set himself ablaze early Sunday morning in Qinghai in northwest China, the official Xinhua news agency reported, saying the monk was a high-ranking lama -- a Tibetan Buddhist spiritual leader.

It is the first time the Tibetan-inhabited province has been hit by such a death. Most self-immolations have taken place in neighbouring Sichuan province, in what rights groups say are protests against perceived religious repression.

The Tibetan government-in-exile, based in India, said in a statement the victim, whom it called Sonam Wangyal, self-immolated in Darlag county in the Tibetan autonomous prefecture of Golog to protest a lack of religious freedom.

"Due to his position as a local spiritual leader, approximately 2,000 local Tibetans are said to have held a candlelight vigil urging the local police authorities to release his body," it said.

It added local police "averted further tension" by agreeing to do so -- a move confirmed by Xinhua, which said his body had been returned to relatives so they could hold a funeral in keeping with local customs.

Discounting the theory that the monk's death was a protest, Xinhua said an initial investigation concluded he killed himself after his affair with a married woman was discovered by her husband.

The incident comes after two former monks in Sichuan set themselves on fire on Friday -- one died and another is being treated in hospital.

The two -- 18 and 22-years-old respectively -- had once been monks at Kirti monastery, the scene of sporadic bouts of unrest since a young monk named Phuntsog set himself alight last March.

The United States issued a statement saying it was "seriously concerned" by the self-immolations, adding that they represent deep frustration over the crackdown on religious freedom in China.

"We're seriously concerned by reports that three more Tibetans have self-immolated over the past few days," State Department spokeswoman Victoria Nuland said, adding that Washington has consistently raised the issue with China.

"These actions clearly represent... enormous anger, enormous frustration with regard to the severe restrictions on human rights, including religious freedom, inside China," Nuland said.

The exiled Dalai Lama has condemned self-immolation, which many Buddhists believe is contrary to their faith, but said recently Tibetans faced "cultural genocide" under hardline Chinese rule that he blamed for the protests.

Xinhua previously reported that police believe some of the self-immolations were "masterminded and instigated by the Dalai Lama clique, which had plotted a chain of self-immolations in the past months for splitting (separatist) motives."

The Dalai Lama, who fled Tibet following a failed uprising against Chinese rule in 1959, founded the government in exile in the northern Indian town of Dharamshala after being offered refuge there.

He remains revered in China's Tibetan areas but is vilified as a "separatist" by China's communist authorities.

Many Tibetans in China accuse the government of religious repression and eroding their culture, as the country's majority Han ethnic group increasingly moves into historically Tibetan areas.

But China rejects this, saying Tibetans enjoy religious freedom and pointing to huge ongoing investment which it says has brought modernisation and a better standard of living.

One woman working in a grocery shop in a town in Darlag, who wished to remain anonymous, said she had heard about the self-immolation.

"I am Han and not long ago I came here to do business," she told AFP.

"I don't know why he burned himself... but we generally have a good relationship with the locals."