Air strikes and artillery have pounded rebel-held districts of eastern Aleppo for a second day, killing dozens of people, damaging medical facilities and flattening residential buildings.
Sources told Al Jazeera on Wednesday that at least 84 people had been killed and dozens wounded in and around Aleppo city over the past 48 hours, as air raids launched by Syrian jets rained down on rebel-held areas amid a new government offensive.
Syria's war: Panic as air strikes target rebel-held Aleppo
The Syrian Observatory for Human Rights said at least 21 people, including five children and an emergency worker, were killed on Wednesday in the al-Shaar, al-Sukkari, al-Sakhour and Karam al-Beik neighbourhoods.
"The helicopters won't stop for a single moment," Bebars Mishal, a member of the Syrian Civil Defence, a first responder group that operates in rebel-held areas, told Reuters news agency.
"Right now, the bombing won't let up."
Al Jazeera's Osama bin Javaid, reporting from Kilis on the Turkish side of the Turkey-Syria border, said casualty figures were "rising by the hour".
"One of the worst-hit areas is the al-Shaar neighbourhood where at least 18 barrel bombs struck, one of them near a children's hospital and a school," he said.
"We have seen disturbing scenes of residents holding up body parts of relatives killed in air strikes," he added.
"So what we're seeing on the ground is utter desperation on the part of the people of Aleppo who are not sure what to do because they can't actually go into shelters, and if they stay at home they are at risk of being bombed."
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Adham Sahloul, of the Syrian American Medical Society, which supports several hospitals in opposition areas, said it appeared the government was focusing its fire on Aleppo's medical infrastructure.
There are only five functioning trauma facilities left in eastern Aleppo, Sahloul told the Associated Press news agency.
The Independent Doctors Association, which supports several facilities in Syria, said eastern Aleppo's central blood bank was struck in Wednesday's attacks, as well as a children's hospital supported by the association.
"Me and my staff and all the patients are sitting in one room in the basement right now," a pediatrician who identified himself only as Dr Hatem posted in a note that was cited by the association. "We will try to get out when the air strikes leave our sky. Pray for us please."
OPINION: I live in Aleppo, under siege
The impact on east Aleppo's densely packed civilian neighbourhoods has been devastating. More than 250,000 civilians are still trapped in rebel-held areas, with dwindling food supplies and extremely limited medical care.
"There were scenes of desperation when there was a brief pause in the air strikes," Al Jazeera's Javaid said. "Residents in one area raided a food bank - people are desperate and hungry."
The Syrian army cut the last supply route into rebel-held territory in July and the last time supplies reached east Aleppo was in August.
Aleppo has become the fiercest front in Syria's five-and-a-half-year war, pitting President Bashar al-Assad, supported by Russia, Iran and Shia militias, against Sunni rebels, including groups backed by Turkey, the United States and Gulf monarchies.
The resumption of government air strikes on besieged east Aleppo began just as Russia also announced a wide-scale operation against opposition target in Syria's north and central Homs province. 
Bomb run Aleppo - People & Power
Moscow has denied reports that its jets hit Aleppo in the renewed wave of bombardment, and said it was sticking to a moratorium on air strikes in Aleppo.
Elsewhere on Wednesday, Turkish-backed Syrian opposition forces inched closer to taking the town of al-Bab, about 35km northeast of Aleppo, from the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, Turkish President Recep Tayyip Erdogan told reporters.
He said the opposition fighters were about 2km from al-Bab.
"The siege is going according to plan," Erdogan said. "There is a resistance there at the moment but I don't think it will last long."
Erdogan also said that US-backed Syrian Kurdish fighters - whom Turkey views as "terrorists", claiming they are an extension of an outlawed Kurdish group in Turkey - would soon leave the town of Manbij, in keeping with a purported US promise to Turkey.