Beijing (China) – A Tibetan-inhabited region of China that has witnessed a wave of protests and the killing of at least four people in security forces’ firing this week, appeared to be under lockdown Thursday and advocacy groups reported police and army vehicles arriving in the area.

Tibetan novice monks from Kirti monastery participate in a candlelight vigil to protest against violence by Chinese police against demonstrators in Tibet, at Dharmsala, India.

Intensifying riots in Seda and Luho counties in China’s western Sichuan province have seen at least four protesters being killed in police firing this week. The area, located in the Ganzi Tibetan Autonomous Prefecture, is a hot spot of Tibetan political activism and has seen a wave of self-immolations, mostly by Buddhist monks, to protest against Beijing’s rule over their homeland.

The violent protests come ahead of a landmark visit to the U.S. next month by Chinese Vice President Xi Jinping, who is set to succeed Hu Jintao as president in a leadership change this year.

On Thursday, police cars and buses and a line of military trucks were seen winding their way up the mountainous road towards Luhuo and Seda from the provincial capital of Chengdu, according to AFP.

Phone lines too appeared unavailable Thursday.

AFP said its contacts in Luhuo such as monks at Drakgo Monastery – located just one kilometre away from the scene of Monday’s protest – were unavailable and calls made to many hotels, restaurants and other places too could not be connected, suggesting phone lines in the town may have been disabled, it said.

In Seda also calls to hotels and restaurants could not be connected. Advocacy groups said Tibetans were not allowed to move freely in Seda. The London-based advocacy group Free Tibet said at least two Tibetans were killed in clashes in Seda on Tuesday and many others injured. On Monday, in similar clashes in Luhuo county, in the Ganzi prefecture, two protesters were killed in firing by police while 36 people were injured, it said.

China’s foreign ministry has said that protesters in both the places were violent. On Tuesday, spokesman Hong Lei accused “overseas secessionist groups” of trying to discredit the government by hyping accounts of what happened.

The US has expressed “grave concern” at the violence, the worst seen in the mountainous region since 2008. Lobsang Sangay, head of the Tibetan government-in-exile in Dharamsala, India, has called for greater international attention on religious and cultural restrictions in the Tibetan-populated regions of China’s west.

“It is high time for [the international community] to intervene to prevent further bloodshed,” Sangay said in a statement. “As a nation aspiring to become a world economic and political power, the People’s Republic of China cannot be permitted to behave in such [an] immoral and violent manner.”

China says that living standards in Tibet have improved drastically since Communist forces took control of the region in 1951. But Beijing has been unable to suppress calls for greater cultural and religious freedom, or loyalty to Tibet’s spiritual leader the Dalai Lama, who fled to India in 1959.