Russian jets have reportedly resumed heavy bombing on the city of Allepo with at least 25 dead and scores wounded.
This was made known by a rebel official and the Britain-based Syrian Observatory for Human Rights monitoring group.
Air strikes mostly hit the Bustan al-Qasr neighbourhood, Zakaria Malhifji of the Aleppo-based Fastaqim rebel group told Reuters.
"There is renewed bombardment and it is heavy," he said.
The Observatory said the death toll from bombing in Bustan al-Qasr, Fardous and other neighbourhoods rose to at least 25, with scores of wounded.
At least 50 civilians were killed by strikes on the rebel-held part of the city and nearby villages controlled by insurgents, residents and rescue workers said. In Bustan al Qasr, residents said, the strikes hit a medical centre and a children's playground.
The Syrian army, backed by Iranian-backed militias, also said it had consolidated its control of the al Jandoul traffic circle at a major road intersection on the northern outskirts of Aleppo.
Moscow and Damascus reduced air raids in the northern city last week. The Syrian army said that was partly to allow civilians to leave opposition-held eastern neighbourhoods.
The Syrian government said rebels holed up in Aleppo can leave with their families if they lay down their arms. Insurgents denounced that offer as a deception.
President Bashar al-Assad seeks the complete recapture of Aleppo, Syria's biggest city before the 5 1/2-year war, which has been divided between government and opposition control for years.
Assad's Russian allies have meanwhile built up its forces in Syria after a brief ceasefire collapsed last month.
Since Russia intervened in the war a year ago, the government's side has gained the upper hand on numerous fronts, including Aleppo, where the opposition-held sector has been completely encircled for weeks.
Insurgents have advanced elsewhere, including in Hama province further south, where they captured a series of towns and villages last month. But in recent days government forces have regained some of that ground.
In the southern city of Deraa, which is split between government and rebel control, insurgent shelling of a school killed at least five people, including children, on Tuesday, the Observatory and state media reported.
Rebels denied they fired at the school. Residents reported the same death toll.
Near the Turkish border, rebels backed by Turkey and a U.S.-led coalition closed in on the Islamic State-held village of Dabiq, the site of an apocalyptic prophesy central to the militant group's ideology.
Free Syrian Army (FSA) rebel groups have been pushing south into Islamic State's territory in an operation backed by Turkey since Aug. 24, and have taken more villages near Dabiq in recent days.
But hundreds of mines planted by the militants were delaying their progress, rebels said. The militants even retook the villages of Ihtimlat and Kafra only hours after the FSA fighters seized them, rebels said.
"They planted along their front lines of defence hundreds of mines," a rebel from the Failaq al Sham group said. Now the goal of FSA forces was to retake the town of Soran, an Islamic State stronghold in the area, before moving on to Dabiq, he said.