Intense government air strikes and artillery fire killed at least 27 people on Saturday in rebel-held parts of Syria's Aleppo, where hospitals have been destroyed and schools forced to close.
An AFP correspondent described relentless bombardment with air strikes, mortar rounds and barrel bombs slamming into residential neighbourhoods in the east of the battered second city.
"It is a catastrophic day in besieged Aleppo with unprecedented bombardment with every type of weapon," a member of the White Helmets rescue group said in a video posted on the organisation's Facebook page.
"People went to sleep to the sound of bombardment and awoke to the sound of bombardment," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a Britain-based monitor of the war.
Syrian government troops began a new assault on the rebel side of Aleppo on Tuesday, as they renewed their bid to recapture the east of the city.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that has killed more than 300,000 people since it began with anti-government protests in March 2011.
The city has been divided between government control in the west and rebel control in the east since mid-2012.
More than 250,000 people remain in the opposition-held part of the city, which has been besieged by the regime since July.
The intensity of the bombardment has forced residents to stay indoors, leaving streets all but deserted.
"There's barely a neighbourhood that has been spared," said Abdel Rahman.
"People don't dare to leave their homes."
Schools in east Aleppo, many of which already operate from basements because of government attacks, announced in a statement they would close Saturday and Sunday "for the safety of students and teachers, after the barbarous aerial strikes".
The bombardment has badly affected rescue and medical facilities in the east, which have already routinely been targeted in government attacks.
On Friday, regime shelling partly destroyed one of the last hospitals in the east, killing two patients and forcing it to shut.
The last paediatric hospital in the east was also forced to close after being hit in multiple attacks, with medical staff evacuating babies from incubators and transferring them to a new location.
"This is a dark day for east Aleppo," said Teresa Sancristoval, emergency coordinator for the Doctors Without Borders (MSF) charity.
"The severity of the bombing has inflicted huge damage on the few hospitals working around the clock to provide medical care."
A centre belonging to the White Helmet rescue group in the Bab al-Nayrab district was also totally destroyed in an air strike on Friday, an AFP correspondent said.
The White Helmets have struggled to keep up with calls for help since the renewed bombardment began, at times unable to leave their centres because of the intensity of the government fire.
"We have no more (body) bags," said one rescue worker in another video posted Saturday by the group from Aleppo.
The Observatory said at least 27 civilians were killed in east Aleppo on Saturday alone, with the toll likely to rise because of the number of seriously wounded.
These deaths brought the toll since the renewed assault began on Tuesday to 92, with state media saying two people were also killed in rebel fire on the government-held west of Aleppo on Saturday.
The assault on rebel-held Aleppo has ended a period of relative respite after regime ally Russia halted its strikes and organised a series of brief truces intended to convince residents and surrendering rebels to leave.
Syria expert Thomas Pierret said regime forces "intended to combine air strikes with famine resulting from the siege to get rebels to surrender".
"Aleppo is now completely besieged and its residents are starting to die of hunger," he said.
Elsewhere, an alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces said they had seized the hilltop village of Tal Saman from the Islamic State (IS) militant group.
The village is just 25 kilometres (15 miles) from IS's de facto Syrian capital of Raqa, which the SDF began an operation to recapture earlier this month.
The SDF is being supported by the US-led coalition fighting IS, which is carrying out air strikes, but has also stepped up deliveries of weapons and equipment to the alliance, its commanders told AFP.