A hospital, blood bank and ambulances are reported to have been hit as Syrian government aircraft and artillery bombarded rebel-held eastern Aleppo.
The staff of a children's hospital in the Shaar district were forced to take shelter in the basement.
Activists say at least 32 people, including children, have been killed in Aleppo over the past two days.
The air strikes resumed on Tuesday after a three-week moratorium declared by the government's ally Russia ended.
Activists confirmed the resumption of government air strikes, amid reports by state media of large troop deployments on several fronts ahead of a major ground assault.
The UK-based Syrian Observatory, a monitoring group, said missiles fired by jets, barrel bombs dropped from helicopters and artillery targeted several areas of eastern Aleppo on Wednesday, including Shaar, Sukkari, Sakhour and Karam al-Beik.
At least 21 people, including five children and an emergency worker, were killed, the group said.
The Independent Doctors Association reported that a children's hospital it supported had been badly damaged.
It quoted the hospital's director, Dr Hatem, as saying he was trapped in the basement. "The planes are up above. We can't get out. Maybe we can protect ourselves in this room," he added.
Rescue workers from the Syria Civil Defence, who are also known as the White Helmets, meanwhile confirmed a paramedic had been killed in Karam al-Beik.
"The helicopters won't stop for a single moment," spokesman Bebars Mishal told the Reuters news agency. "Right now, the bombing won't let up."
Air raids also reportedly continued in the countryside west of Aleppo. The Syrian Observatory said 19 people were killed in the village of Batbo on Wednesday.
Russia also announced the start of a major operation against jihadist militants elsewhere in western Syria that saw the country's only aircraft carrier used in combat for the first time.
Aleppo, once Syria's commercial and industrial hub, has been divided in roughly two since 2012, with the government controlling the west and rebels the east.
In the past year, Syrian troops have broken the deadlock with the help of Iranian-backed militias and Russian air strikes.
On 22 September, two weeks after encircling the east and imposing a siege on its 275,000 residents, they launched an all-out offensive to take full control of the city.
The government and its ally Russia halted air strikes on 18 October to allow civilians and rebels to leave, but few took up the offer.
By the end of the month, air strikes and shelling had killed more than 700 civilians in the east, while rocket-fire had left scores dead in the west, according to the UN.