Eight people were killed in a mutiny at a prison in Syria's second city of Aleppo during the night, the opposition Syrian National Council said on Tuesday.
Security forces "opened fire with bullets and tear gas on the detainees at Aleppo central prison in response to a peaceful sit-in organised by prisoners because of the great injustice of which they are victims," the SNC said in a statement.
"Eight people were martyred and a fire broke out inside the prison."
The statement said regime forces had fired on the prison from helicopters as the fire raged inside, preventing help from arriving at the facility.
Aleppo has been the scene of fierce fighting since Friday, with the country's second city emerging as the new front in the battle between rebel fighters and the regime of Syrian President Bashar al-Assad.
The Syrian Observatory for Human Rights watchdog group said clashes raged through the night, with heavy fighting reported in several neighbourhoods and at least four people killed, including one rebel fighter.
Elsewhere in the country, the Local Coordination Committees, which organises opposition forces on the ground, reported renewed shelling in parts of the capital Damascus, including in the Barzeh district where the neighbourhood's mosque was reported damaged by intense shelling.
The Aleppo mutiny is the second in less than a week in Syria, after a similar incident in the central prison in the southern city of Homs.
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On Saturday, the Syrian Observatory for Human Rights reported that two inmates were killed when troops put down that mutiny.
On Monday, an activist who identified herself as Umm al-Sakher said her sister, a detainee in the Homs prison, was "still living in danger. There is no food, no water in the prison. It is completely surrounded."
Observatory head Rami Abdel Rahman told AFP that, after the mutiny, there were fears some of the Homs detainees might be executed.
"We have reports that after the mutiny, the authorities have sent a group of judges to the prison to investigate. It is possible their presence might lead to summary executions," Abdel Rahman said.
Earlier this month, Human Rights Watch charged that Syria was holding tens of thousands of detainees in a "torture archipelago" in which they were subjected to beatings, electric shocks and other abuse.
The New York-based rights group documented 27 detention facilities across the country it said were used to hold people swept up in the government's brutal crackdown on a 16-month uprising.
The Syrian Observatory says tens of thousands of people have been detained and more than 17,000 killed since March 2011.
Syrian official media say more than 4,000 people have been set free in seven rounds since the start of the uprising, including 1,000 in May alone.