A coalition of Syrian rebels announced a new military operation in Aleppo province on Thursday, where more than 250 people have been killed in regime barrel bomb attacks since Saturday.
The Islamic Front -- a huge alliance grouping tens of thousands of rebels -- and the jihadist Al-Nusra Front announced an operation dubbed "Truthful Promise Approaches," the name a reference to a passage in the Quran.
"All military fighters in bases must go to the frontlines or they will be questioned and held accountable," the groups said in a statement.
It asked residents of "occupied areas" to stay away from government checkpoints and bases, saying they would be targeted.
The announcement comes as the Syrian army seeks to take territory in the rebel-held eastern part of Aleppo city.
Troops are moving from areas around Aleppo international airport after recapturing territory nearby and reopening it to air traffic.
The ground campaign has been accompanied by six consecutive days of aerial attacks involving explosive-packed barrel bombs dropped from army helicopters.
The Syrian Observatory for Human Rights said Thursday that at least 257 people have been killed in barrel bomb attacks on eastern Aleppo since Saturday, including 11 on Thursday.
The dead include at least 76 children, according to the Britain-based group.
Hundreds more have been wounded in the raids using the controversial unguided munitions, which have been condemned by rights groups as indiscriminate.
The heavy casualty toll has sparked a mass exodus from the worst-hit neighbourhoods in the east of the city.
Once the country's economic hub, Aleppo has been divided between government control in the west and rebel control in the east since a rebel offensive in mid-2012.
The bombing in the east has prompted thousands of civilians to flee across the nearby border to Turkey, or seek refuge in the government-held part of Aleppo, with much of the surrounding province engulfed in fighting between rebels and the jihadist Islamic State of Iraq and the Levant.