Fighting for a key military base outside Syria's main northern city of Aleppo killed at least 15 pro-regime militiamen on Thursday, a monitoring group said.
"Fifteen members of the National Defence Forces were killed in fighting against the Islamic State of Iraq and the Levant, Al-Nusra Front and Islamist groups in the east of Aleppo province and near Base 80," the Syrian Observatory for Human Rights said.
Both ISIL and Al-Nusra are loyal to Al-Qaeda and have played a major role in operations on the ground, to the concern of the opposition's Western supporters.
The army recaptured Base 80, which had provided the garrison for Aleppo's military and civilian airports, at the weekend after months of fighting.
It was one of a series of setbacks for the rebels in recent weeks around Syria's main cities of Damascus and Aleppo.
North of the capital on Wednesday, fighting raged in the Qalamoun mountains near the Lebanese border, where a flare-up of violence has sparked a new exodus of refugees.
The mixed Christian-Sunni Muslim area is strategic because it provides a key supply line for rebels around both Damascus and the third city of Homs to its north.
"The humanitarian situation is very bad," an opposition activist in the area told AFP over the Internet.
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"Because of the density of the population here, we fear a real catastrophe."
Thousands of refugees have fled Qalamoun for neighbouring Lebanon in recent days, swamping the small border town of Arsal.
A Syrian military source said there had been fighting in the Qalamoun town of Deir Attiyeh with rebel fighters who had fled the army's recapture this week of nearby Qara.
Troops, meanwhile, pressed an offensive against the last rebel-held districts of Homs. At least 18 people were killed in exchanges of shellfire in the central city, the Observatory said.
The dead included two members of the mukhabarat, the regime secret services, it said.
The SANA state news agency reported that six people in Damascus were wounded on Thursday by shells fired by "terrorists", the regime term for the armed opposition.
More than 120,000 people have been killed in 32 months of conflict ravaging Syria, according to Observatory figures.
It began as demonstrations demanding the fall of President Bashar al-Assad, but spiralled into a civil war after a brutal regime crackdown on dissent.