Jihadist rebels seized control of a prison in the northern Raqa province and freed "hundreds" of detainees overnight, the Syrian Observatory for Human Rights reported on Sunday.
"Government forces pulled out of Raqa's central prison located in the northern part of the provincial capital after clashes that lasted days," the watchdog said.
Jihadist rebels from Al-Nusra Front and other insurgents took over the prison late on Saturday "and liberated hundreds of prisoners," it said, adding some detainees were transferred to nearby Tal Abiad to appear before a local Islamist court.
The prison takeover came as battles raged Saturday on the outskirts of the city of Raqa between rebels and Syrian troops, the Britain-based Observatory said.
Sixteen rebels and 14 soldiers were killed in the fighting, which the Observatory said was "the most violent in the region" since the uprising against President Bashar al-Assad erupted nearly two years ago.
The rebels, who control most of the countryside of Raqa, have been attacking army checkpoint on the outskirts of the provincial capital.
Elsewhere, rebel fighters and government forces fought for control of a sprawling police academy at Khan al-Assal in the northern province of Aleppo, the watchdog said.
On Saturday, rebels stormed the main building of the police academy.
In the oil-rich province of Hassakeh in northeast Syria, Kurdish militants opposed to the Assad regime captured the towns of Ramilan and Qahtaniyeh from government forces, the Observatory said.
At least 156 people were killed in violence across Syria on Saturday, including two Palestinians hanged at the Yarmuk refugee camp in Damascus on suspicion of aiding the regime, it said.
The Observatory, which relies on a network of activists and medics on the ground to collect information, circulated a picture of the hangings.