Russia warned the United States Saturday against carrying out any attacks on Syrian government forces, saying it would have repercussions across the Middle East as regime troops pressed a Russian-backed offensive to retake the entire city.

Russian news agencies quoted Foreign Ministry spokeswoman Maria Zakharova as saying that a US intervention against the Syrian army “will lead to terrible, tectonic consequences not only on the territory of this country but also in the region on the whole.”

She said regime change in Syria would create a vacuum that would be “quickly filled” by “terrorists of all stripes.”

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The largest hospital in rebel-held east Aleppo was bombed on Saturday for the second time in days.

Aleppo, once Syria’s vibrant commercial powerhouse, is now at the heart of a major military campaign by President Bashar 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.

As the situation for civilians grows increasingly dire, the biggest hospital in east Aleppo was hit by two barrel bombs on Saturday, the medical organization 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 and remained trapped there.

SAMS radiologist and hospital administrator Mohammad Abu Rajab made an urgent call for help on Saturday morning 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, known as M2, in what UN chief Ban Ki-moon denounced as “war crimes.”

That bombardment heavily damaged the two facilities and left only six fully-functional hospitals in the city’s east, according to SAMS.

At the bombed hospital, an AFP journalist saw blood-stained hospital beds and dented equipment lying in disarray beneath blown-out windows, while medical staff in green scrubs picked through the rubble outside to assess the damage.

The strikes were strongly denounced by French Foreign Minister Jean-Marc Ayrault.

“In the flood of violence that has overwhelmed Aleppo for several days, the systematic targeting of structures and health workers is particularly unjustifiable,” Ayrault said.

“This new attack only confirms the absolute urgency of a cessation of hostilities in Aleppo and access for civilian populations to the humanitarian assistance they desperately need,” he said.

The World Health Organization has called Syria the most dangerous place in the world 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.

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 SANA official news agency said 13 people were wounded Saturday in the western Al-Midan neighborhood, also by rebel shellfire.

The assault has seen government forces seize territory in both the Suleiman al-Halabi neighborhood in the city center, as well on the northern edges of Aleppo.

On Saturday, regime loyalists backed by heavy raids advanced on the edges of the Bustan al-Basha neighborhood in Aleppo’s north, the Observatory said.

An AFP correspondent said clashes and the loud booms of shelling could be heard around the Suleiman al-Halabi and Bustan al-Basha fronts throughout the night.

Residents of regime-held areas expressed relief that the rebels were being pushed back but said they feared retaliation.

“We were happy when we heard about the army’s advance,” said Majed Abboud, a 32-year-old car dealer.

“But I’m afraid that with these ferocious clashes, there will be some kind of reaction from the armed groups,” he said.

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.”

Diplomatic efforts to put an end to the fighting across the country have all but collapsed.

Russia’s Foreign Minister Sergei Lavrov on Friday accused Washington of protecting jihadist groups in its effort to overthrow Assad.

British Foreign Secretary Boris Johnson said Russia was in danger of becoming “a pariah nation” and the attacks in Aleppo were “unquestionably a war crime.”