Protests in Tibet over Beijing's rule have spread to neighbouring parts of China, after days of demonstrations and violence in Tibet's main city, Lhasa.
Chinese troops patrolled the streets of Lhasa on Sunday
Clashes between Tibetan protesters and police in Aba, Sichuan province, saw a police station and cars attacked.
Rights groups said several people had been killed in the clashes. Protests were also reported in Gansu province.
The unrest came after exiled Tibetan leaders said a Chinese crackdown had killed at least 80 people in Lhasa.
Indian-based officials said the figure was confirmed by several sources, even though China had put the death toll during Friday's riots at 10.
'Rule of terror'
The Dalai Lama has called for an international inquiry into China's crackdown, accusing it of a "rule of terror" and "cultural genocide".
The clashes in Aba, known as Ngawa in Tibetan, happened around 1200 local time on Sunday, according to Kate Saunders of the International Campaign for Tibet.
"According to reliable reports the police opened fire," said Ms Saunders, who is in London but said she had indirect phone and web access to eyewitness accounts. "We know there have been deaths."
She said that more than 1,000 monks had been on the streets of the town, which is home to a large monastery.
Accounts of how many people died differ, but she said the most reliable eyewitness source put the toll at seven.
Reuters news agency cited an unnamed police officer in Aba saying that Tibetans had thrown petrol bombs, burned a police station, and torched vehicles during the clashes.
The BBC has learned that troops in Sichuan province have been recalled from leave and put on standby.
China says Tibet always part of its territory
Tibet enjoyed long periods of autonomy before 20th century
1950: China launched a military assault
Opposition to Chinese rule led to bloody uprising in 1959
Tibet's spiritual leader the Dalai Lama fled to India
In China's north-western Gansu province, at Machu town, hundreds of protesters marched on government buildings and set fire to Chinese businesses, Reuters reported, quoting the Free Tibet Campaign.
An account emailed to the BBC from a witness in the town said about 1,500 people - monks and lay people - shouting "Free Tibet" and "Long Live" the Dalai Lama were tear-gassed by security forces.
In the Gansu capital Lanzhou, more than 100 Tibetan students staged a sit-down protest on a university's playing field, according to the activist group Free Tibet.
Elsewhere in Gansu, at Xiahe, security forces extended their clampdown on Sunday after confrontations there between hundreds of monks and police over the weekend.
In Lhasa, where demonstrators set fire to Chinese-owned shops and hurled rocks at local police on Friday after days of mainly peaceful protests, Chinese troops were out in force.
The authorities in Tibet have urged the protesters to hand themselves in by Monday morning.
In an interview with the BBC, Tibet's spiritual leader the Dalai Lama said he feared there would be more deaths unless Beijing changed its policies towards Tibet.
"It has become really very, very tense. Now today and yesterday, the Tibetan side is determined. The Chinese side also equally determined. So that means, the result: killing, more suffering," he said.
China says Tibet has always been part of its territory, though Tibet enjoyed long periods of autonomy before the 20th Century and many Tibetans remain loyal to the Dalai Lama, who fled in 1959.
The unrest erupted a fortnight before China's Olympic celebrations kick off with the start of the torch relay, which is scheduled to pass through Tibet.
The Dalai Lama emphasised that he still supported Beijing's staging of the Olympic Games this summer, saying it was an opportunity for the Chinese to show their support for the principle of freedom.