Regime aircraft pounded rebel areas of Syria's second city Aleppo on Friday, hitting a clinic just days after a strike destroyed a hospital, killing two doctors and sparking an international outcry.
More than 200 civilians have been killed in Aleppo over the past week as rebels have pounded government-held neighbourhoods with rocket and artillery fire and the regime has hit rebel areas with air raids.
The bloodshed has brought a February 27 ceasefire between government forces and non-militant rebels to the verge of collapse and raised fears of a humanitarian crisis in the northern metropolis and other battleground areas.
Air strikes on Aleppo hospital kill 27, including children
A nurse was among several people wounded when the air strike hit the clinic in the rebel-held Al-Marja neighbourhood, the civil defence known as the White Helmets said.
The clinic, which had been providing dental services and treatment for chronic illnesses for about five years, was badly damaged.
An AFP photographer said he heard nearly a dozen air raids within the space of a few minutes, followed by the wail of ambulances.
"The planes didn't sleep and didn't let us sleep either," one resident of the densely populated Bustan al-Qasr district told AFP.
"The earth is shaking beneath our feet."
At least two civilians were killed in the strikes on Friday, one of them a child, the Syrian Observatory for Human Rights said.
The rebels bombarded government-held areas with rocket and artillery fire, killing three people as they were leaving a mosque after the main weekly prayers, state television reported.
In rebel areas, Friday prayers were cancelled because of the air strikes.
It was the second time this week that an air strike had hit one of the few medical facilities still operating in rebel areas.
Late on Wednesday, air strikes hit the Al-Quds hospital and a nearby block of flats in the Sukkari neighbourhood, killing 30 people, including one of the last paediatricians still working in the east of the city.
Air strikes hit MSF-backed hospital in Syria, 9 dead
US Secretary of State John Kerry expressed "outrage" over the hit on the hospital, saying it appeared to be "a deliberate strike on a known medical facility."
He called on Moscow to press its Damascus ally "to stop attacking civilians, medical facilities, and first responders, and to abide fully by the cessation of hostilities."
Al-Quds was supported by both Doctors Without Borders and the International Committee of the Red Cross.
The ICRC has warned that Aleppo is "on the brink of humanitarian disaster".
"Everyone here fears for their lives and nobody knows what is coming next," said Valter Gros, who heads the ICRC's Aleppo office.
Thursday was the deadliest day in Aleppo since the violence flared last week, with 54 civilians killed, according to the Observatory.
"It is the worst day in Aleppo in five years. The regime did not spare a single neighbourhood," one resident told AFP.
'Aleppo is burning'
An online campaign to halt the carnage picked up speed, with Twitter users posting pictures of destroyed buildings in flames with the hashtag #AleppoIsBurning.
UN Secretary-General Ban Ki-moon said attacks that target civilians are "inexcusable" violations of humanitarian law.
"There must be accountability for these crimes," he said.
Aleppo was once Syria's economic powerhouse, but it has been ravaged by fighting since 2012 when rebels seized the east of the city, confining the government to the west.
The worsening bloodshed has raised fears for the ceasefire in other areas of Syria and called into question the future of peace talks in Geneva that have now gone into recess.
In what would probably be the death knell for the tattered truce, the Syrian army is poised to launch an offensive to retake the whole of Aleppo and the surrounding province.
Leading pro-government daily Al-Watan said it would begin in the next few days.
"Now is the time to launch the battle for the complete liberation of Aleppo," the paper said, adding that it "will not take long to begin, nor to finish".
23 die of starvation in beseiged Syrian town: MSF
Control of Aleppo province is divided between a myriad of warring armed groups -- militants of Al-Qaeda and the Islamic State group, Kurdish militia and various rebel factions as well as the army.
The province is heavily fought over because of its supply lines to neighbouring Turkey.
Since the conflict in Syria erupted in 2011, more than 270,000 people have been killed and millions more been forced from their homes.