US Secretary of State John Kerry said in a telephone call to his Russian counterpart, Sergey Lavrov, that the United States and its partners held Russia responsible for a drastic escalation in attacks that have put civilians at great risk.
Two hospitals in eastern Aleppo have been bombed "out of service," staff and activists said, as airstrikes pounded rebel-held parts of the northern Syrian city on Wednesday.
Hospital staff said the city's M2 and M10 hospitals were hit early Wednesday, putting intensive care units out of use.
Dr. Mounir Hakimi of the International Union of Medical Care and Relief Organizations and UK-based charity Syrian Relief told CNN that an airstrike hit the M10 hospital and shelling the M2.
"Anyone offering help to civilians now is being targeted," he said. "It's the message that any humanitarian response is not welcome."
At least six people were killed Wednesday morning when an airstrike struck a bakery as people lined up to buy bread, according to activists. They said the death toll was expected to rise.
Ground battles also raged Wednesday in Aleppo between the Syrian army and rebels.
The Syrian army said it took control of the central Farafra neighborhood in Aleppo's old city Tuesday, bringing it closer to the rebel-held eastern areas.
However, rebels and activists inside the besieged city told CNN there had been no ground assault in those areas.
There are conflicting reports of how many people were killed Tuesday in airstrikes on eastern Aleppo.
The rebel-held Aleppo Health Directorate reported 18 deaths, including six children. The activist group Aleppo Media Center said more than 30 people were killed, but it was unclear if some of those deaths were a result of earlier strikes or bodies pulled out of the rubble.
The UN children's agency, UNICEF, said Wednesday that at least 96 children had been killed and 223 injured in eastern Aleppo since Friday.
Hundreds of airstrikes have rocked the beleaguered city, home to more than 250,000 people, since the Syrian government, backed by Russia, announced a renewed, "comprehensive" offensive on Thursday.
Hospital strikes 'a big blow'
The strikes on two of Aleppo's remaining hospitals likely will only compound the misery of civilians trapped there.
Hakimi, a UK-based orthopedic surgeon, told CNN that due to damage to the electricity and oxygen generators and the intensive care unit at M10, "the hospital as we speak is out of service. It's one of the main hospitals in Aleppo, so it's a big blow."
There were no casualties in the strike on M10, Hakimi said, but three patients were killed and two members of staff were injured at the M2 hospital. Five artillery shells struck that facility, he said, citing information from medical and humanitarian workers on the ground.
"The problem now is that there's no intensive care units in either hospital," Hakimi said. "They were already running at double capacity because during the weekend there was the most intensive airstrikes ever."
The remaining hospitals are already running at full capacity, he said. There may be more instances in which doctors perform surgery on patients as they lie on the ground due to lack of space, he said.
"We are so close to humanitarian catastrophe in Aleppo," Hakimi said.
The Syria Civil Defense group, a volunteer emergency medical service also known as the White Helmets, was also hit in Aleppo on the weekend, he said.
UNICEF said the 30 doctors who remain in Aleppo face an increasing number of trauma cases with hardly any medical equipment or medicine to treat the injured.
The agency cited a doctor on the ground as saying that children with low chances of survival are often left to die because of a lack of capacity or supplies.
"The children of Aleppo are trapped in a living nightmare," said UNICEF Deputy Executive Director Justin Forsyth. "There are no words left to describe the suffering they are experiencing."
Pope urges peace
On Wednesday, Pope Francis again appealed for peace in Syria and warned that those responsible for the bombing campaign in Aleppo will be "accountable to God" for their actions, according to Vatican Radio.
"In expressing my deep sorrow and lively concern for what is happening in that already battered city -- where children, the elderly, the sick, young and old, all are dying -- I renew my appeal to everyone to commit themselves with all their strength to the protection of civilians as an imperative and urgent obligation," Francis said, speaking at his general audience in Rome.
More than 200 airstrikes hit Aleppo over the weekend, killing more than 100 people and injuring hundreds more, according to Ammar al-Selmo of the Syria Civil Defense group.
On Sunday, top UN officials described the Syrian regime's offensive against areas of Aleppo as "barbaric."