China has jailed three Tibetan monks for between 10 and 13 years for helping a fellow monk to burn himself to death, sparking criticism from the United States and rights groups. A court in the southern province of Sichuan handed long jail sentences to two more monks on Tuesday over the self-immolation protest at their monastery, the state Xinhua news agency reported. The sentences, for "intentional homicide" for having "plotted, instigated and assisted" in the death, followed the jailing Monday of a Tibetan lama from the same monastery for 11 years. The monk's death at the flashpoint Kirti monastery in a mainly Tibetan area of Sichuan triggered months of protests, and Human Rights Watch said the sentences appeared to be a sign of growing government frustration. Phelim Kine, a senior Asia researcher for the New York-based campaign group, said this appeared to be a "harsher punishment" indicating the "ratcheting up of China's central government's objectives". "The objectives result from the increasing frustration with the Kirti monastery's refusal to buckle under central government authority," "The sentencing shows the central government is sending a message that the gloves are off." The Kirti monastery is in Sichuan's Aba prefecture, the scene of repeated anti-government protests. The US State Department, commenting on Tuesday after the first sentence, questioned whether China had followed either international or its own legal standards in the case. "We urge Chinese leaders to address policies in Tibetan areas that have created tension and to protect Tibetans' unique linguistic, cultural and religious identity," it added in a statement. Many Tibetans in China are angry about what they view as increasing domination by China's majority Han ethnic group and accuse the government of trying to dilute their predominantly Buddhist culture. China, however, says that Tibetan living standards have improved markedly in recent decades, pointing to the billions of dollars it has spent on infrastructure and development projects. The Xinhua report said that the lama, identified only as Drongdru, had hidden the injured monk, Phuntsog, for 11 hours and ensured emergency treatment did not reach him. It said the monks had sent photos of Phuntsog overseas via the Internet three days before he burnt himself to death, "proving that the self-immolation was premeditated". The agency said all three monks had confessed, and quoted what it said were monks who attended the trial condemning their actions. Repeated calls by AFP to the Sichuan court went unanswered. The Xinhua account contradicted those of rights activists who have asserted that monks at the monastery rescued Phuntsog from the police, who beat the monk after extinguishing the flames. Bequelin of Human Rights Watch called Drongdru's conviction "purely political". "It comes against a background of unprecedented persecution against the monastery of Kirti, from where the government has already taken into arbitrary detention dozens of monks," Bequelin told AFP. The Kirti monastery has remained extremely tense since security forces shot dead several protesters in March 2008, Bequelin added. "Sentencing a monk who appears to have only attempted to protect Phuntsog after his solitary act only compounds the agony for Kirti monks," said Kate Saunders of the International Campaign for Tibet. "By doing so the Chinese government aims to deflect attention from the real reasons for the self-immolation, which was an expression of anguish and sacrifice due to intense repression including new measures to suppress religious practice in Tibetan areas." Xinhua said that Phuntsog was just 16 years old at the time of his death, but the International Campaign for Tibet put the monk's age at 20. Another monk killed himself by self-immolation in August at Sichuan's Nyitso monastery, drinking petrol before setting himself alight. Soldiers and police responded by surrounding the monastery.