Syrian troops have clashed with rebels in the capital, Damascus, and the second city, Aleppo, as they try to retake areas out of their control.
In Aleppo and Homs, there have been revolts and attempted breakouts by prisoners. Nine inmates died during the Aleppo jail rebellion, activists say.
Activists say security forces are threatening to storm the Homs prison.
The renewed fighting comes amid strong international concern over Syria's threat to use chemical weapons.
Damascus said on Monday that such weapons would not be deployed inside Syria but would be against foreign attack.
Explosions and fires have been reported from the Homs jail, where unarmed policemen are said to have defected and prisoners have staged a sit-in. The Syrian Observatory for Human Rights said two prisoners had been killed.
The activist group the Local Co-Ordination Committee (LCC) said there was a "catastrophic humanitarian situation" inside the jail.
The group said the military was planning a massacre and called for international help to prevent "mass executions" of prisoners.
Government officials had earlier denied there had been a defection.
Syrian state television says the government has now regained control of most areas of Damascus which had been captured by rebels last week.
It has broadcast footage of the southern district of Nahr Isha, showing the bodies of "terrorists" it says were killed in the fighting.