Two children were among at least 13 people killed in new air strikes on a rebel-held district of Syria's main northern city of Aleppo on Tuesday, a watchdog said.

It was the third straight day of air raids on the eastern Shaar neighbourhood of the city, Syria's commercial hub before a rebel offensive in July last year made it a key battleground in the conflict which erupted in March 2011, the Syrian Observatory for Human Rights said.

A woman was also among the dead, the Britain-based watchdog said.

The new raids came after air strikes on rebel-held areas of Aleppo on Sunday killed 76 people, among them 28 children. Strikes on Monday killed a further 10 civilians, among them four children.

The Syrian air force has been dropping barrels of explosives on rebel-held areas in its bombing campaign, which has been carried out by helicopter as well as fixed-wing aircraft, activists and the Observatory say.

A Syrian security official said the military prefers the TNT-packed barrels because they are cheaper than regular bombs, which need to be imported from Russia.