The largest hospital in rebel-held east Aleppo was bombed for the second time in days as Syrian government forces pressed a Russian-backed offensive to retake the entire city.
Aleppo, once Syria's vibrant commercial powerhouse, is now at the heart of a major military campaign by President Bashar al-Assad's fighters and his steadfast ally Moscow.
The offensive, announced on September 22, has seen dozens of civilians killed and residential buildings flattened in the east, where an estimated 250,000 people live under government siege.
Diplomatic efforts to end the fighting across the country have all but collapsed.
However, the foreign ministers of the United States and Russia, which brokered a week-long truce deal that collapsed last month, spoke by phone on Saturday.
The foreign ministry in Moscow said on Facebook that Sergei Lavrov spoke to his American counterpart John Kerry and they "examined the situation in Syria, including the possibility of normalising the situation around Aleppo".
It said "illegal armed groups" continue fighting in the city despite Russian-US agreements.
And Russian Foreign Ministry spokeswoman Maria Zakharova warned the US against taking any direct action against Damascus or the Syrian army.
"It will lead to terrible, tectonic shifts not only on the territory of this country but also in the region in general," she said, according to the Russian state-controlled news provider Sputnik.
As the situation for civilians in Aleppo grows increasingly dire, the largest hospital in the east of the city was hit by barrel bombs on Saturday, the medical organisation that supports it said.
"Two barrel bombs hit the M10 hospital and there were reports of a cluster bomb as well," said Adham Sahloul of the Syrian American Medical Society (SAMS).
Sahloul said a small group of patients and doctors "were inside the hospital for basic triage, bandaging, and cleaning services for emergency cases" when the bombardment began.
- 'SOS, everyone!' -
SAMS radiologist and hospital administrator Mohammad Abu Rajab made an urgent call for help from inside M10.
"The hospital is being destroyed! SOS, everyone!" he said in an audio message distributed to journalists.
M10 had already been hit on Wednesday along with the second-largest hospital in the area, M2.
That bombardment badly damaged the two facilities and left only six fully-functional hospitals in east Aleppo, according to SAMS.
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At the bombed hospital, an AFP journalist saw bloodstained hospital beds and dented equipment lying in disarray beneath blown-out windows.
"A new barrel bomb fell this afternoon in front of the hospital, forcing medical staff... to evacuate all patients to another one and leave the hospital," a doctor at M10 told AFP.
European Parliament president Martin Schulz called the hospital bombing a "war crime", tweeting that the international community "must unite to prevent city annihilation".
French Foreign Minister Jean-Marc Ayrault said "the systematic targeting of structures and health workers is particularly unjustifiable".
The six-nation Gulf Cooperation Council urged the UN Security Council to "immediately intervene to stop the aggression against Aleppo".
The World Health Organization has called Syria the world's most dangerous place for health workers, and Aleppo in particular has seen much of its medical infrastructure destroyed or heavily damaged.
Since fighting first broke out there in 2012, Aleppo has been divided by a front line between rebel forces in the east and government troops in the west.
- Water cuts -
After the government launched its offensive last month, more than 220 people have been killed by bombardment on Aleppo's east, including six children and 12 other civilians on Friday, according to the Syrian Observatory for Human Rights.
In west Aleppo, rebel rocket fire killed 15 civilians and wounded 40 on Friday, state television reported. The official news agency SANA said 13 people were wounded Saturday in the western Al-Midan neighbourhood, also by rebel shellfire.
The assault has seen government forces seize territory in both the Suleiman al-Halabi neighbourhood in the city centre and on the northern edges of Aleppo.
On Saturday, regime loyalists advanced on the edges of the Bustan al-Basha neighbourhood in Aleppo's north, the Observatory said.
An AFP correspondent said clashes and the loud booms of shelling were heard around the Suleiman al-Halabi and Bustan al-Basha fronts throughout the night.
An official at the Suleiman al-Halabi pumping station in the rebel-held area said most of Aleppo had water cuts on Saturday because of damage.
Residents of regime-held areas expressed relief that the rebels were being pushed back, but said they feared retaliation.
The battle for Aleppo has sparked some of the most brutal violence since the March 2011 beginning of Syria's conflict, which has killed more than 300,000 people and displaced over half the population.
The Doctors Without Borders (MSF) medical charity warned Friday that "bombs are raining" over the city, turning east Aleppo into "a giant kill box".