Chinese officials cut off mobile phone and internet connections to areas where Tibetans were shot dead amid unrest last month, state media has reported.

Officials say security forces fired in self-defence after mobs of rioters attacked police and official buildings in the south-western province of Sichuan, resulting in two deaths.

Tibetan exiles and campaign groups say police fired at peaceful protesters and killed at least three people.

It has been impossible to verify accounts of the unrest. Foreign reporters attempting to visit the region have been turned back, with officials blaming bad weather and the state of the roads.

Friday's English-language edition of the Global Times newspaper published a report from the region titled Monks Run Amok. The police chief of Luhuo in Ganzi – a county known to Tibetans as Drango – said police first tried to disperse rioters with high-power water guns and rubber bullets, but failed.

"Rioters continued to attack and tried to grab the guns from the police," he told the paper. "[Officers] first shot in the air as a warning, but it was completely ignored, so we had no other choice but to open fire."

The newspaper said the incident began with a protest that became violent. It said other Tibetan-populated counties had quickly tightened security, allowing police to quickly control the next day's unrest in Seda, known to Tibetans as Serthar, where another Tibetan was shot dead.

"After the riots, internet connections and mobile phone signals were cut off for over 50km [30 miles] around the riot areas. Police believe external forces played a part in the riots," the newspaper said.

In 2009, China cut off internet and text messaging services across the north-western region of Xinjiang after ethnic riots in the capital, Urumqi, left almost 200 dead.

Officials blamed "trained separatists" for instigating the events in Ganzi. They have also sought to blame outsiders for a string of self-immolations by Tibetan clergy and laypeople over the last year, mostly in Sichuan.

China appears to have stepped up security across other Tibetan areas, with the top party official in Lhasa urging security forces to increase surveillance of monasteries and main roads in the Tibet Autonomous Region.

Qi Zhala ordered all people entering Tibet to carry identification cards from March, the state news agency Xinhua reported. He urged officials to strive for "no big incidents, no medium incidents and not even a small incident".