Syrian forces shelled rebel-controlled zones of Aleppo before dawn on Wednesday, killing at least 19 people, among them seven children, the Syrian Observatory for Human Rights said.
Ten civilians were killed in the southern neighbourhood of Bustan al-Qasr while a total of nine bodies, including those of the children, were found in the Marjeh and Hanano neighbourhoods, the Britain-based watchdog said.
Activists have reported relentless bombardments and food shortages in rebel-held neighbourhoods of the country's commercial capital, while an AFP reporter said life in the centre of the city was relatively normal.
Rebels fighters on July 20 opened a new front in the Syrian conflict by launching an attack on Aleppo.
The army has since dislodged them from several sectors, including one of their main strongholds Salaheddin, but pockets of resistance remain.
On Monday, a senior commander in charge of the government's military assault on the rebels in Aleppo predicted his forces would recapture the city within 10 days.
New UN-Arab League envoy Lakhdar Brahimi said on Tuesday the death toll in Syria was "staggering" and the destruction "catastrophic."
The former Algerian foreign minister, who took up his post on Saturday, also warned that the situation across the country was "deteriorating steadily."
His comments came as International Committee of the Red Cross chief Peter Maurer launched a mission in Syria to seek greater protection for civilians and the UN refugee agency said 100,000 people had fled the country in August alone to seek refuge in neighbouring states.
The Observatory, which relies on information from a network of activists on the ground, says more than 26,000 people have been killed overall in Syria since the revolt against President Bashar al-Assad's rule broke out in March 2011.