Pro-Tibet groups reacted with incredulity on Friday to Chinese reports that police had merely wounded four people after opening fire "in self-defence" during unrest in southwestern Sichuan province.

"At this point any statement the Chinese government puts out has virtually no credibility," said Lhadon Tethong from Students for a Free Tibet, based in Dharamsala, India.

"We are seeing photographs, we have friends who have lost relatives. We categorically reject any of the (official Chinese) information."

Kate Saunders, from the International Campaign for Tibet, said the Chinese version of events in Sichuan was as unbelievable as the Tibetan authorities' assertion that no guns were fired in putting down violent protests in Lhasa.

"Tibetans have taken tremendous risks to continue to let us know what is happening," she told AFP from London.

"They have provided compelling evidence that there were deaths. I do not think the world believes (the Chinese version) at this point."

The Xinhua report, which quoted police sources, marked the first time China has admitted using potentially lethal force to quell unrest that first erupted in Tibet last week and spilled into adjacent provinces including Sichuan.

The shootings occurred in the remote Tibetan-populated county of Ngawa, or Aba in Chinese, amid protests that broke out there last Sunday following deadly rioting in Tibet itself.

Activist groups, however, have said at least eight people were killed by security forces in Ngawa and circulated photos of dead bodies with apparent bullet wounds.

The veracity of the photos could not be independently verified by AFP.

Chinese authorities said 13 "innocent" people were killed in the violence in Lhasa, while the Tibetan government-in-exile says at least 99 have been killed.

Several tourists and witnesses have said they heard gunfire during the riots.

The protests started in Lhasa to mark the anniversary of a failed 1959 uprising against Chinese rule, which had begun in 1951 after communist troops moved into the Buddhist region to "liberate" it.