At least 12 people were killed and 30 wounded on Monday in rebel fire on government-held districts of Syria's second city Aleppo, state television said.
In a breaking news alert, the channel said children were among the casualties in "terrorist shelling of the city".
The bombardment was also reported by the Syrian Observatory for Human Rights, a Britain-based monitor, which put the death toll at 13, including five children.
The group reported at least 33 people wounded and said the death toll was expected to rise because of the number of seriously injured.
The Observatory said the rocket fire hit several neighbourhoods in the government-controlled west of Aleppo, and came a day after at least four people were killed in similar rebel fire.
Once Syria's industrial powerhouse, Aleppo has been ravaged by the conflict, and control of the city is divided between the government-held west and rebel-held east.
Government forces regularly pound rebel-held areas from the air, and opposition fighters fire rockets and mortar rounds into the west.
The rebel fire came as Human Rights Watch criticised opposition fighters in a new report for "indiscriminate" attacks against civilians in government-controlled areas.
HRW warned there had been a "race to the bottom in Syria, with rebel groups mimicking the ruthlessness of government forces".
Elsewhere, the Observatory said five children from the same family were killed in a government barrel bomb attack on a town in the southern Syrian province of Daraa.
Barrel bombs are crudely made weapons -- barrels or oil drums packed with explosives -- that are usually dropped by helicopter and have been criticised as indiscriminate by rights groups.
More than 215,000 people have been killed in Syria since an anti-government uprising that began with peaceful demonstrations in March 2011.
The conflict has since become a complex multi-front war that has attracted thousands of foreign fighters to jihadist organisations such as the Islamic State group.
Daesh claimed responsibility on Saturday for a double bomb attack on Kurdish new year festivities in the northeastern city of Hasakeh on Friday night.
On Monday, the Observatory said the toll in that attack had risen from 45 to 54 dead, among them 20 children.