Bomb attacks have hit rebel-held eastern Aleppo for a second straight day, pounding a district that houses several medical facilities, including the central blood bank.
The attacks forced Syrian staff and patients in the only remaining paediatric hospital to cower in a basement as buildings collapsed around them.
At least 54 people were killed in air strikes and artillery shelling across northern Syria, part of a long-anticipated offensive against rebel-held areas announced by Russia, a key ally of Syrian President Bashar Assad.
The bombardment hit in besieged neighbourhoods of Aleppo, as well as the surrounding countryside and the nearby rebel-held province of Idlib.
Russia said its air raids were only targeting Idlib and the central province of Homs to root out militants of the Islamic State group and Syria's al Qaida affiliate.
But Syrian war planes were pounding rebel-held districts of Aleppo, home to nearly 275,000 people.
Residents said the aerial campaign intensified on Wednesday. Syria's Civil Defence, known as the White Helmets, recorded at least 150 raids, more than double the number of attacks on Aleppo on Tuesday.
Resident Modar Shekho said: "The helicopters would leave and the jets would arrive," adding that the helicopters were dropping seven or eight "barrel bombs" at a time "causing a lot of destruction".
The head of the only paediatric hospital still in service in eastern Aleppo described taking shelter in a basement with some 50 young patients and staff for more than two hours as the bombs and other missiles fell around them.
"It was frightening," said Dr Hatem. He said he counted more than 20 missiles falling, and a number of nearby buildings were destroyed.
When he and the others emerged, he said they found missiles had landed in the hospital's courtyard, hit the main door and stairs. The hospital's operating rooms, incubators and other equipment were moved underground four months earlier because of repeated aerial bombings of the city.
"We have no other way to reinforce the hospital. We see barrel bombs bringing down whole buildings," he said. "We don't really have many options."
Adham Sahloul, of the Syrian American Medical Society, which supports several hospitals in opposition areas in Syria, said it appeared the government was focusing its fire on Aleppo's medical infrastructure, including the central blood bank, which was also hit. There are only five functioning trauma facilities left in eastern Aleppo, he said.
The managing director of the blood bank, Ahmad Eid, said the damage was mainly to the facility's exterior and reception area. The driver was slightly injured, but the stored blood was not affected, he said.
"This was directly targeting the medical quarter. It is a very vital area," Mr Eid said.
The Independent Doctors Association, a Syrian group which supports the children's hospital and the blood bank, decried the lack of protection of civilians as the conflict rages.
"Aleppo has been under siege since July and the escalating bombardment on the eastern part of the city has rendered the medical mission nearly impossible," the group said in a statement.
Medical facilities have repeatedly come under attack in the conflict. The World Health Organisation said it has documented with its partners 126 attacks on medical facilities across the country this year, including five hospitals that were struck in Aleppo and Idlib between Sunday and Tuesday alone.
Months of negotiations between Moscow and the Obama administration have failed to cement a long-term cease-fire in Aleppo, which has become the focus of the war between Mr Assad and rebels fighting to topple him. Al Qaida's Syrian affiliate is fighting alongside the rebels, but the Islamic State group has no presence in Aleppo.
The Russian declaration of the offensive came hours after President Vladimir Putin and US President-elect Donald Trump discussed Syria in a phone call and agreed on the need to combine efforts in the fight against what the Kremlin called their number one enemy - "international terrorism and extremism".
In an interview broadcast on Tuesday with Portugal's state-run RTP television, Mr Assad accused armed groups he called "terrorists" of occupying eastern Aleppo and refusing government offers to evacuate. He said his mission was to liberate civilians.
Mr Assad also identified President-elect Mr Trump as a possible "natural ally", if he turned out to be "genuine" about his commitment to fight terror in Syria. Mr Trump has indicated he would prioritise defeating the Islamic State group in Syria over regime change, saying the rebels could be "worse" than the sitting president.
A strict blockade of rebel-held areas of Aleppo has been enforced since July, and talks to allow in food and medical supplies have failed. The UN warned last week that food rations inside Aleppo's rebel-held districts could be depleted by the end of this week.
In a sign of the rising desperation, a local Aleppo official said riots broke out outside a warehouse storing grain and other food on Tuesday and Wednesday, leading to clashes that left one person dead.
The official said the rioters stormed and emptied the warehouse. A resident of the area confirmed the warehouse was robbed.
The Britain-based Syrian Observatory for Human Rights said air strikes in Aleppo killed at least 24 people, including six children. At least one paramedic was among the dead, according to the Syrian Civil Defence.
In northern Idlib province, air strikes hit near a school in al-Habeet, killing two children from the same family, the observatory said. Another seven were killed elsewhere in Idlib. Rami Abdurrahman, the observatory's director, said the air strikes in Idlib were believed to be Russian.
In rural Aleppo, air strikes in the village of Batbo killed at least 21, including eight women and three children, many from the same families, when they hit traditional mud houses. It was not clear who was behind those air strikes.