Fierce clashes broke out Wednesday on the edge of the northern city of Aleppo, a day after rebels overran a nearby air base, while a deadly blast rocked Raqa city, a watchdog said.
"Fierce clashes pitting rebels... against regime forces broke out near the airforce intelligence building in the Layramun area on the edges of Aleppo," said the Syrian Observatory for Human Rights.
The monitoring group said the rebels engaged in the battle were Islamists, among them the jihadist Al-Nusra Front.
The battle comes a day after rebels overran the air base at nearby Minnigh.
An activist in Aleppo said rebels were trying to advance towards Nabul and Zahraa, majority Shiite villages in the Aleppo countryside that have turned into bastions of President Bashar al-Assad's regime.
Most of Syria's rebels -- like the population -- are Sunni, while Assad belongs to the Alawite community, an offshoot of Shiite Islam.
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"Some 300 fighters who had been fighting for Minnigh are now free to fight elsewhere," said Mohammed, an activist with the anti-regime Aleppo Media Centre.
Elsewhere, an explosion ripped through the northern city of Raqa, Syria's only provincial capital to have fallen out of regime control, killing three people including two children, said the Observatory.
The Britain-based group said the origin of the explosion has not been identified, and added that dozens of people were wounded.
Near Damascus, an army ambush killed and wounded "dozens" of rebels" near the strategic industrial city of Adra, which lies at the northeastern entrance to the capital.
The area has seen frequent violence, as rebels try to take control and use it as a launch-pad for operations inside Damascus.
Nearby, the air force staged two air strikes on the battered town of Douma, where anti-regime sentiment has been strong since early in the anti-Assad revolt, which broke out in March 2011.
The UN says more than 100,000 people have been killed in Syria's war, and millions more forced to flee their homes.