Police in China shot dead more protesters in Tibetan-inhabited areas as unrest spread, official media and an advocacy group said Wednesday, amid signs of a physical and virtual lockdown on the region.

China's Sichuan province, which has big populations of ethnic Tibetans, many of whom complain of severe repression, has been rocked by violent clashes this week -- some of the worst since huge protests against Chinese rule in 2008.

The United States on Tuesday said it was 'seriously concerned' by the situation, calling on Chinese security forces to 'exercise restraint' and urging authorities to allow journalists and diplomats into flashpoint areas.

There were indications on Wednesday that the authorities were restricting movement and communications in the region as unrest spread.

Phone numbers that previously worked were suddenly unavailable and one local said his town had been closed off by authorities.

The situation has prompted Lobsang Sangay, head of the India-based Tibetan government-in-exile, to call on the international community 'to not remain passive' and 'to intervene to prevent further bloodshed'.

The unrest comes at a time of increasing tensions in Tibetan-inhabited areas, where at least 16 people have set themselves ablaze in less than a year -- including four this month alone -- over a lack of religious freedom.

In the latest bout of violence, police opened fire on a protest in Seda county on Tuesday, killing two protesters and injuring scores of others, according to the exiled Tibetan government and advocacy group Free Tibet.

The official Xinhua news agency, citing local authorities, said police had shot dead only one 'rioter' and injured another, adding they had to use lethal force after a violent mob attacked them with knives, gasoline bottles and guns.

The government, police and locals in Seda would not comment.

Free Tibet Director Stephanie Brigden disputed the official account of Tuesday's violence, saying, "There has never been any evidence of any Tibetan protest where knives have been used".

"Of all the freedom movements across the globe the Tibetan movement is probably the most well known non-violent struggle," Brigden said.

The incident came a day after security forces shot at a crowd of Tibetans protesting against religious repression in the nearby town of Luhuo, killing at least two and injuring more than 30, according to locals and rights groups.

Monks reached by phone on Monday and Tuesday at the Drakgo Monastery in Luhuo said police shot at the crowd through the windows of the police station, and wounded people had taken refuge in the monastery as thousands of armed officers stood guard outside.

China's foreign ministry, however, said the Luhuo protesters were also violent. On Tuesday, spokesman Hong Lei accused 'overseas secessionist groups' of trying to discredit the government by hyping accounts of what happened.

By Wednesday, AFP was unable to telephone anyone in Luhuo -- not even police and government offices -- as calls were met with a rapid beeping tone, suggesting phone lines in the town may have been disabled.

Phone calls to hotels and restaurants in neighbouring Daofu county met with the same ring tone.

The few people AFP was able to contact described a huge security presence and restrictions on people's movements.

In Aba county, where Free Tibet says another protest erupted on Monday but was soon quelled by police with tear gas, a hotel worker said there were 'lots of armed forces and armed vehicles on the street.'

"No outsiders can come freely into Aba. People who want to come in must be screened and show valid ID," the man, who refused to be named, said. Another hotel employee in the same place confirmed the big security presence.

Beijing insists Tibetans enjoy freedom of religious belief and says their lives have been made better by huge ongoing investment into Tibetan-inhabited areas.

It blames the Dalai Lama -- who fled Tibet in 1959 after a failed uprising against Chinese rule -- for fomenting much of the unrest in a bid to split Tibet from the rest of China, which the Buddhist spiritual leader denies.

On the Tibetan government-in-exile's website, prime minister Sangay said it was 'high time for (the international community) to intervene to prevent further bloodshed' following the unrest this week.

"How long and how many tragic deaths are necessary before the world takes a firm moral stand?" he said.

On Tuesday, Maria Otero, special coordinator for Tibetan issues at the US State Department, said Washington had repeatedly urged China to address 'counterproductive policies in Tibetan areas'.