At least 20 people were killed and 100 wounded on Monday in Syrian rebel rocket fire on government-held parts of the city of Aleppo, Syrian state media said, according to AFP.
"The number of martyrs in the massacre carried out by terrorists in Aleppo has risen to 20 dead and more than 100 hurt," state television said in a breaking news alert.
The Syrian Observatory for Human Rights reported the rocket fire as well, putting the toll at 13, including two children, and adding that it was expected to rise.
The group also said around 100 people were wounded, among them 20 children.
The Britain-based monitor said rebels had fired some 250 rockets at several districts in western Aleppo city in just four hours, causing massive damage including the collapse of an entire building.
Aleppo, formerly Syria's economic powerhouse, has been ravaged by the conflict that began with anti-government protests in March 2011, before spiralling into a civil war.
The city has been divided between government control in the west and rebel control in the east since shortly after fighting spread to it in mid-2012.
In addition to the rebel attacks, regime forces regularly bombard the eastern side from the air, dropping crude barrel bombs that rights groups say are indiscriminate and can kill dozens of civilians at a time.
Rights groups have similarly criticized rebels for firing indiscriminately into civilian areas in western Aleppo.
The rebel attack came as the UN's peace envoy to Syria, Staffan de Mistura, arrived in Damascus for talks with President Bashar Al-Assad.
The envoy is expected to raise with Syrian officials what his spokesman described as the "unacceptable" use of barrel bombs by the regime.
De Mistura previously called the civilian deaths in barrel bomb attacks "totally unacceptable". Assad, however, has repeatedly denied using barrel bombs and has suggested in interviews that no such weaponry exists.
More than 230,000 people have been killed in Syria since March 2011.