Syrian government troops and rebels have clashed around a prison in the flashpoint northern city of Aleppo.
Reports suggest the rebel fighters may have tried to blow up the walls of the prison, which holds some 4,000 inmates.
Activists said government forces had counter-attacked using tank shells and air raids.
Meanwhile, the rebel Free Syrian Army (FSA) vowed to punish criminals after a video appeared apparently showing a rebel biting a dead soldier's organs.
In Aleppo, rebels appear to have detonated car bombs outside the walls of the prison on Wednesday morning, according to the Syrian Observatory for Human Rights (SOHR), a UK-based activist group.
Government sources claimed to have fought back, injuring and killing opposition fighters.
Clashes were still continuing early on Wednesday afternoon, according to a BBC reporter in Damascus.
The FSA put out a statement saying its field commanders had been instructed to "to begin a prompt investigation" into the video in which a well-known insurgent from the city of Homs, Abu Sakkar, is shown apparently cutting out the soldier's heart.
"Any act contrary to the values that the Syrian people have paid their blood and lost their homes to will not be tolerated, the abuser will be punished severely even if they are associated with the Free Syrian Army," the FSA said, according to AFP news agency.
"The perpetrator will be brought to justice," it said.
In the video, which cannot be independently authenticated, Abu Sakkar is shown standing over the soldier's corpse, saying: "I swear to God we will eat your hearts and your livers, you soldiers of Bashar the dog."
US-based Human Rights Watch said his actions were a war crime.
Also on Wednesday, a US-based web monitoring company said that Syria appeared to be experiencing a nationwide internet blackout for the third time in six months.
Renesys Corporation said that Syrian internet services had gone offline at 10:00 local time (07:00 GMT).
The UN says nearly 80,000 people have been killed since the uprising against President Assad began in March 2011, and millions have fled their homes.