The death toll from what residents said an attack with barrel bombs steadily rose throughout the day as bombardment rocked the city, according to the Syrian Observatory for Human Rights.
"Eleven civilians, including four children, were killed by air raids after midnight in the Bab al-Nasr area of Old Aleppo, and seven others were killed in Fardous neighbourhood," the monitor said.
At least six people, including a child and two women, were killed in the Salhin district, in addition to four more civilians in other rebel-controlled neighbourhoods, the Britain-based monitor said.
"At least 20 people are still under the rubble," said Observatory head Rami Abdel Rahman.
Syrian state news agency SANA, for its part, reported that one person was killed and nine others were wounded in rebel rocket fire on government-controlled parts of the city.
An AFP correspondent in eastern Aleppo said helicopters and fighter jets were circling rebel-held neighbourhoods, adding that barrel bombs -- crude, unguided explosive devices -- had been dropped on several areas.
A hospital in the Maadi neighbourhood was hit in the bombing, wounding some of the staff and patients inside.
"All kinds of weapons were used to bomb the hospital, from midnight until about 11:00 am. Now it's unusable," Mohammad Kheir, one of its doctors, told AFP.
"There were some injuries among the medical staff but thankfully they are only light wounds."
A crying woman clad in a black robe desperately grasped the leg of a bloodied young man as doctors treated him on the hospital floor.
The Observatory said rebel fighters shelled government-controlled western areas of Aleppo, but had no immediate word on any casualties.
Aleppo city is divided roughly between government control in the west and rebel control in the east.
It was once Syria's commercial powerhouse but has since been ravaged by the country's five-year war.
