Nineteen Syrian government soldiers and security officers were killed on Monday when rebels detonated explosives in a tunnel under their positions in the northern city of Aleppo, a monitoring group said.
Rebels also launched some 300 shells on the divided city's government-held western sector, killing at least five people and wounding 80 others, the Syrian Observatory for Human Rights said.
Rebels in Aleppo and nearby areas have repeatedly destroyed government positions by tunnelling underneath them to lay explosives. Some such attacks have caused dozens of casualties.
The violence comes after government forces all but surrounded rebel-held eastern Aleppo on Thursday, effectively imposing a siege on the estimated 250,000 to 300,000 residents remaining there.
Some 33 people were killed in government airstrikes in the Aleppo region and neighbouring Idlib province, the Observatory said.
In the village of Abian, west of Aleppo, 11 children were among 16 people killed in a government air raid late Saturday.
In Tramaneen village in Idlib province 14 people were killed in an airstrike, among them three rescue volunteers and an activist, the Observatory said. Three more were killed in the Idlib village of Ahsem.
Aleppo, Syria's largest city before the war, has been divided since fighting erupted for control of the city in mid 2012.
The rebel-held western sector has been devastated by repeated shelling and air raids, often hitting civilian targets including hospitals.
On Thursday government forces cut off the Castello Road, the last route into the rebel-held east. There has been heavy fighting in the area since then as rebels attempt to recapture the road.
Syrian state media said that their troops managed to foil a wide-scale rebel counter-attack on Sunday.
According to the Observatory, almost 800 civilians have been killed since tit-for-tat shelling and airstrikes by rebels and government forces intensified in late April.
Of those, 444 were killed in government airstrikes and shelling on eastern Aleppo, and 325 in rebel shelling of the government-held western sector.
Another 23 died in rebel shelling on the Kurdish-held Sheikh Maqsoud district, while six were killed by Kurdish snipers on or around the Castello Road, which passes close to Sheikh Maqsoud.
The Syrian conflict, which began with peaceful protests against the regime of President Bashar al-Assad, has turned into a four-way war between the government, mainly Islamist rebels, the Islamic State extremist group and leftist Kurdish forces.
Some 4.8 million Syrians have fled the country since 2011, with another 6.6 million displaced inside its borders, according to the latest UN estimates.
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