A Tibetan-inhabited region of China appeared to be under lockdown Thursday after it was rocked by deadly clashes, as exile groups gave grisly details of how the unrest unfolded.

The west of Sichuan province, which has big populations of ethnic Tibetans, many of whom complain of repression, was earlier this week hit by some of the worst unrest since huge protests against Chinese rule in 2008.

Security forces fired into two separate crowds of protesters in Luhuo and Seda towns on Monday and Tuesday in the remote, rugged prefecture of Ganzi, which borders Tibet.

Advocacy groups say at least three were killed in the clashes but maintain the protests were peaceful until police opened fire. China says two died -- one in each incident -- and acknowledged police shootings only in Seda.

Lobsang Sangay, head of the exiled Tibetan government, said six Tibetans had reportedly been killed and around 60 injured -- some critically -- in the violence, and condemned the shootings as "gruesome acts".

In a statement, he urged Tibetans not to celebrate Losar -- Tibetan New Year -- which this year falls on February 22, and also asked people around the world to participate in a vigil on February 8.

Under the apparent lockdown Thursday in affected areas of Ganzi, phone calls did not go through, the Internet was cut off and movement restricted as security forces poured into the region, locals and advocacy groups said.

Even in the provincial capital of Chengdu -- some 600 kilometres (370 miles) from Luhuo -- police cars were parked every 50 metres (yards) in the Tibetan quarter.

"It's forbidden to take photos or to interview people," one officer told AFP reporters.

But the US-based International Campaign for Tibet (ICT), which has local and exiled contacts, was still able to glean details from sources of what happened in Seda on Tuesday.

The official Xinhua news agency, citing local authorities, said one "rioter" was killed and another injured and that police had to resort to lethal force after a violent mob attacked them with knives, gasoline bottles and guns.

ICT, however, said hundreds of Tibetans had gathered peacefully in the town square and that after some time, armed police fired tear gas and started shooting into the crowd.

"Tibetans were running everywhere to escape... Some couldn't run away because they were too seriously injured," the group quoted an exile source with contacts in the area as saying.

Other sources said the square was "covered in blood" with tear gas canisters scattered in the street.

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

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.

The unrest comes at a time of rising tensions in Tibetan-inhabited areas, where at least 16 people have set themselves ablaze in less than a year -- including four this month alone -- prompting an increase in security.

On Thursday, dozens of police cars and buses were seen winding their way up the snowy, mountainous road towards Luhuo and Seda from Chengdu, AFP reporters witnessed.

"They had come down to Chengdu to celebrate the new year, but have to go back before the end of the holiday due to the unrest," said Zhou Ming, a driver who often takes the same route.

Zhou said he was unable to call any of his friends in the affected areas on their mobiles or fixed lines.

Contacts in Luhuo such as monks at Drakgo Monastery, which is located just one kilometre away from the scene of Monday's protest, who spoke to AFP earlier in the week were also unavailable on Thursday.

Calls made to 19 different hotels, restaurants, companies and shops in Luhuo were met with a rapid beeping tone, suggesting phone lines in the town may have been disabled.

Calls to 15 hotels and restaurants in Seda were also met with the same ring tone although AFP was able to reach a man at the local government who said the unrest had died down but added there were armed police and vehicles patrolling.

Advocacy groups said Tibetans were not allowed to move freely in Seda and that the area was now completely locked down, with some sources reporting at least 40 military trucks arriving in the town.

AFP reporters trying to travel to the affected areas were stopped from going forward at a checkpoint some 330 kilometres away from Luhuo, where police checked people in every vehicle.