Russia warned the United States about taking direct action against the Syrian regime, saying it would cause negative consequences across the Middle East, Russia's state-run Sputnik news agency reported Saturday.
Meanwhile, intense fighting continued in the besieged Syrian city Aleppo on Saturday, with one of the main hospitals bombed by Russian-backed Syrian forces.
Sputnik reported that Russian's Foreign Ministry spokeswoman Maria Zakharova said, "If the US launches a direct aggression against Damascus and 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 the United States risked creating a power vacuum in Syria were it to depose Syrian President Bashar al-Assad. Such a power vacuum would be filled by "terrorists of all sorts," Sputnik reported.
Secretary of State John Kerry, in a meeting with a group of Syrian civilians last week, expressed sympathy for their demands that the United States intervene more forcefully amid Syrian and Russian airstrikes, according to an audio recording obtained by CNN.
He told the group that he "lost the argument" for using military force against the regime of Syrian President Bashar al-Assad.
"I've argued for the use of force. I'm the guy who stood up and announced that we're going to attack Assad for the use of weapons," Kerry is heard telling the Syrian attendees, referring to internal deliberations within the administration of President Barack Obama that followed Assad's use of chemical weapons in 2013.
In Aleppo, the M10 hospital was hit Saturday for the second time in four days.
The facility was struck by a "torrent" of weapons including two barrel bombs, two cluster bombs and at least one rocket, said Adham Sahloul, spokesman for the Syrian American Medical Society.
One person died and 15 were wounded in the bombings, according to an activist with the opposition-aligned group Aleppo Media Center. Some patients hurt in the attack were also wounded when bombs struck the same facility Wednesday, the activist said.
M10, the largest surgical hospital in Aleppo, is now out of service, the activist said.
The hospital had just reopened to offer basic emergency care Friday, Sahloul said, following the airstrike earlier in the week that also shut down its desperately needed intensive care unit. The city's M2 hospital was also put out of service by shelling Wednesday, activists said. The Syrian American Medical Society supports both hospitals.
The attacks on M2 and M10 have left only two surgical hospitals in Aleppo, the media center activist said.
On Friday alone, the M10 hospital saw 84 cases, including 22 children, Sahloul said. Sixteen of those died, including five children.
Aleppo's medical services are under appalling pressure. About 30 doctors remain in eastern Aleppo, Sahloul said, for a population of some 300,000 at a time of urgent need.
Doctors have resorted to triage, prioritizing those they believe have the best chance of survival, the activist said.
Another three medical facilities in al-Shaar neighborhood — a women's hospital, a children's hospital and the central blood bank — were also hit Friday, Sahloul said.
More than 450 people have been killed since a US-Russia brokered ceasefire collapsed September 22, he said.
Syrian government war jets have targeted gathering places such as markets, hospitals and mosques for three days, the Aleppo Media Center activist told CNN.
The United Nations' aid chief Stephen O'Brien repeated a call for a 48-hour weekly pause in the fighting so that aid could enter the city.
"The health system is on the verge of total collapse, with patients being turned away and no medicines available to treat even the most common ailments," O'Brien said in a statement.
"Hundreds of critical medical evacuations are urgently required. With clean water and food in very short supply, the number of people requiring urgent medical evacuations is likely to rise dramatically in the coming days."
Most in the city's eastern districts lack access to clean water following infrastructure damage from shelling and bombing, Sahloul said.
Aleppo saw more aerial bombardments and clashes Saturday following the deaths of 36 people the day before, UK-based monitoring group Syrian Observatory for Human Rights said.
At least 20 people, including six children, were killed then in airstrikes on neighborhoods of rebel-held eastern Aleppo, the group reported.
Separately, at least 16 people, including women and children, were killed Friday in shelling by rebel forces on regime-held parts of western Aleppo, it said.
The state-run Syrian Arab News Agency, or SANA, said 13 people died Friday in Aleppo due to "terrorist rocket attacks."
Rebel-held districts of Aleppo have suffered intense aerial bombardment by Syrian and Russian warplanes for more than a week, while the Assad regime prepares to take the northern city.
Regime forces shelled areas of Aleppo's Old City on Saturday, the Syrian Observatory for Human Rights said. Regime forces and rebel fighters also engaged in violent clashes in the neighborhoods of Bustan al-Basha and Sheikh Khodr.
A Syrian family leaves the area following a reported airstrike on Friday, September 23, in rebel-held east Aleppo. Following the airstrike, recovery teams from Syria Civil Defense, known as the White Helmets, began working to free the trapped and recover the dead, including small children.
Aleppo Media Center said at least one person was killed and several wounded Saturday in airstrikes by helicopters in eastern Aleppo's Al-Sakhour neighborhood.
The group also reported artillery shelling by regime forces.
SANA, quoting a police official, reported that at least 13 people were injured in rocket fire in the al-Maydan residential neighborhood in regime-held western Aleppo.
Meanwhile, an estimated 10,000 Syrian-led troops have gathered in advance of what is believed to be a possible final ground assault by Syrian forces against rebels in Aleppo.
The past week's assault on rebel-held areas of the key city involved some of the worst violence since the start of the war in 2011.