Chinese paramilitaries have killed at least two more Tibetans during protests and detained about 100 others, campaign groups have reported, as the US expressed "grave concern" at some of the worst violence seen in the mountainous region since 2008.

The clashes this week in Ganzi prefecture, known in Tibetan as Kandze, may have claimed five lives since the start of the lunar new year on Monday, and there are concerns that demonstrations against Beijing's rule could spread more widely.

The Chinese government confirmed the unrest, but said police acted to restore order after violent protests. This is denied by Lobsang Sangay, the head of Tibet's government in exile, who has appealed to the international community "to intervene to prevent further bloodshed."

The conflict follows at least 16 self-immolations by monks and lay Tibetans over the past year, mostly in Sichuan. Protesters have sacrificed themselves as a mark of resistance against Beijing's control of their homeland and restrictions on freedom of religion and movement. Many have called for the return of the Dalai Lama, the exiled Buddhist leader revered by many Tibetans.

Tibetan groups refused to celebrate the lunar new year on 23 January as a form of protest. The Tibetan new year is on 22 February.

Demonstrations have erupted in several areas of Ganzi, Sichuan province. Details are hard to confirm independently as many of the areas have been locked down by the authorities.

Free Tibet said on Tuesday that at least two Tibetans were shot dead and many more wounded when Chinese security forces opened fire on protesters in Seda township, known in Tibetan as Serthar. The area is said to be under curfew.

Radio Free Asia – which was founded by the US – reported Chinese authorities killed five Tibetans and wounded 40 others.

"A kind of martial law has been imposed," a local resident, calling himself Ganta, told RFA.

China's state Xinhua news agency confirmed a second day of violence in Ganzi on Tuesday, but it claimed only one demonstrator was killed and 14 policemen were injured.

The previous day, it said one demonstrator was killed in Luhuo (Drango), also in Ganzi. "The crowd, some wielding knives and hurling stones, attacked a police station. They smashed two police vehicles, two fire engines and stormed into nearby shops and a bank, damaging the bank's cash dispenser," the agency said.

The International Campaign for Tibet said three people were killed in clashes and has released photographs of a Tibetan named Yonden, who was reportedly shot by police in Luhuo. His funeral will be held in the coming days, when more protests are expected.

During the protests on Monday, Tibetan sources say police beat isolated demonstrators with batons and rounded them up in trucks. Others were reportedly arrested in a door-to-door sweep. About 100 Tibetans are believed to have been taken into detention, including Lama Pema Dorjechang and other monks from Namtso.

There was also said to have been a roundup of monks in Kirti monastry, Aba (Ngaba) on Monday that prompted a peaceful gathering of several hundred people.

Tensions are thought to be running high in the area after a series of self immolations, a harsh crackdown and the recent early release of a monk – Losang Khedrup – who was reportedly paralysed from the waist down as a result of beatings in prison.

Since December, there have been protests in three of the five provinces of China that are home to significant Tibetan communities. International human rights activists say tensions have never fully eased since the large-scale Tibetan protests in March 2008.

"This is getting worse and worse. It's a vicious circle," said Nicholas Bequelin, of Human Rights Watch. "Beijing appears to have made the calculation that they have no need for Tibetan loyalty anymore."

The US special co-ordinator for Tibetan Issues, Maria Otero, expressed "grave concern" at the upsurge of violence.

"The US government repeatedly has urged the Chinese government to address the counterproductive policies in Tibetan areas that have created tensions and that threaten the distinct religious, cultural and linguistic identity of the Tibetan people," she said.

The Free Tibet director, Stephanie Brigden, called for support from the international community to prevent further bloodshed.

"This is the second consecutive day that Chinese forces have opened fire and killed unarmed Tibetan protesters. Chinese forces are responding with lethal force to Tibetans' ever-growing calls for freedom. Information from Tibet suggests there will be more protests in the coming days and we have grave concerns over what may unfold."