Islamist Al-Nusra Front rebels killed seven Syrian soldiers at a checkpoint in the northern city of Safireh on Friday, after losing more than 100 men in the area over the past 72 hours, a watchdog said.
The checkpoint was guarding a heavily fortified military factory in the south of the city, the Syrian Observatory for Human Rights said, adding that a convoy of reinforcements arrived after the attack.
"At least 112 rebels have been killed since Wednesday in fighting with troops between Safireh and the town of Khanasir" about 40 kilometres (25 miles) farther south, Observatory director Rami Abdel Rahman told AFP by phone.
He said regime forces were stationed at defence establishments in Safireh and have set up checkpoints on the route to Khanasir, but Al-Nusra and several other rebel battalions control the streets of Safireh.
The watchdog said the city has become a "ghost town", after residents fled army bombardments en masse.
The Observatory, which gathers its information from a network of activists and medics in civilian and military hospitals on the ground, gave an initial toll of 45 people killed nationwide on Friday, including 18 civilians.
Elsewhere, troops shelled rebels on the eastern and southern outskirts of Damascus amid street battles as an army offensive raged.
The army on Tuesday launched a major assault on rebel zones surrounding Damascus as it sought to break the stalemate in Syria's almost 23-month conflict.
The Observatory said three rebels were killed on Friday in the southern district of Tadamun while six rebels died in shelling on Moadamiyet al-Sham, near the key Al-Mazzeh military airbase southwest of the capital.
The Zamalka neighbourhood, also in the east, was bombed by warplanes, the watchdog said.
Troops also shelled the eastern city of Deir Ezzor, the northeast province of Raqa, the northwest province of Idlib, the central province of Hama, the northern province of Aleppo and the southern province of Daraa.
The Observatory meanwhile updated its toll from a Wednesday attack on a bus carrying employees from a military-owned factory in Hama province, saying at least 54 civilians, including 11 women, were killed.
"This plant has nothing to do with weapons. They simply sew uniforms and make boots for soldiers," Abdel Rahman said.
The United Nations says more than 60,000 people have been killed in Syria since the outbreak of a revolt in March 2011 that morphed into an insurgency after the army launched a brutal crackdown on dissent.