At least 19 civilians, including five children, were killed by a Syrian regime barrel bomb attack and rebel rocket fire in Aleppo, a monitoring group said on Thursday.

Fifteen of the dead, among them four of the children, came in the barrel bomb strike on a rebel-held district of the divided northern city, the Syrian Observatory for Human Rights said.

"The children were aged under 10," said Observatory chief Rami Abdel Rahman, adding that a pregnant woman was also among Wednesday's dead.

The crude explosives-filled container hit a building in the northeastern neighbourhood of Karam al-Beik as residents were sitting down to the iftar meal, which breaks the daytime fast observed during the Muslim holy month of Ramadan.

The other four civilians were killed by rebel rocket fire on the government-held district of Midane, the Observatory said.

For the past three years, Syria's pre-war economic hub has been divided between a government-held western sector and a rebel-held eastern one.

The rebels launched a major offensive in the first week of July to try to advance the front line.

More than 220,000 people have been killed in Syria since the conflict began with anti-government protests in March 2011 that were met with a bloody crackdown.