More than 280,000 people have been killed and millions displaced since the civil war erupted.
At least 25 civilians including four children were killed and 120 others wounded on Friday in rebel shelling of regime-held districts of Syria's Aleppo, the Syrian Observatory for Human Rights said.
The shelling comes as a 72-hour nationwide ceasefire -- announced by the army to mark the end of the holy month of Ramadan -- is due to end at midnight (2100 GMT Friday).
Fighting has continued since the truce was announced on Wednesday, particularly in and around Aleppo, with deaths on both sides of the divided city.
State news agency SANA said that 20 people had been killed and a further 140 wounded in the shelling of districts in the government-controlled west, and accused the rebels of violating the ceasefire.
An AFP correspondent in the city's rebel-held east said that regime air strikes and rocket fire had also targeted opposition neighbourhoods on Friday.
The Observatory, a Britain-based monitoring group, said that at least four civilians had died in regime air strikes there.
Further north, the army pressed its advance to retake the rebels' sole supply route to Aleppo in heavy fighting.
"The rebels' violent shelling comes as a response to the advance of regime forces towards the Castello road", Observatory chief Rami Abdel Rahman said.
The Syrian army on Thursday advanced within firing range of the supply route, which they have been trying to cut for more than two years.
The road wraps around Aleppo's eastern and northern edges then leads into rebel-controlled territory north of the battered city.
The city -- which was the country's pre-war commercial capital -- has been divided since mid-2012.
More than 280,000 people have been killed and millions displaced since the civil war erupted with the brutal repression of anti-government protests in 2011.