At least 25 people were killed in northwestern Syria in airstrikes carried out by the Syrian government and Russia, the Syrian opposition-run rescue service known as the White Helmets said early on Monday.

Russian and Syrian jets struck the rebel-held city of Idlib in northern Syria on Sunday, military sources said, as President Bashar al-Assad vowed to crush insurgents who had swept into the city of Aleppo.

The army also said it had recaptured several towns that rebels had overrun in recent days.

Residents said one attack hit a crowded residential area in the centre of Idlib, the largest city in a rebel enclave near the Turkish border where around four million people live in makeshift tents and dwellings.

At least seven people were killed and dozens injured, according to rescuers at the scene. The Syrian army and its ally Russia say they target the hideouts of insurgent groups and deny attacking civilians.

Ten children were among the dead in the airstrikes in and around Idlib and other targets in rebel-held territory near Aleppo on Sunday, according to the White Helmets.

At least 20 children killed, says rescue service

The total death toll from Syrian and Russian strikes since Nov. 27 had climbed to 56, including 20 children, the group added in a statement on X.

Reuters could not independently confirm the battlefield accounts.

The insurgents are a coalition of Turkey-backed mainstream secular armed groups along with Hayat Tahrir al-Sham, an Islamist group that has been designated a terrorist outfit by the U.S., Russia, Turkey and other states.

WATCH | Airstrike hits Aleppo hospital: 

Inside an Aleppo hospital after a deadly Russian military airstrike

A Russian airstrike at the Aleppo University Hospital in Syria killed at least 12 people and injured more than 20 others. The bombardment, in support of Syrian government forces, followed insurgents seizing control of Aleppo, the country's largest city.

In a joint statement, the United States, France, Germany, and Britain urged "de-escalation by all parties and the protection of civilians and infrastructure to prevent further displacement and disruption of humanitarian access."

The insurgents seized control of all of Idlib province in recent days, the boldest rebel assault for years in a civil war where front lines had largely been frozen since 2020.

They also swept into the city of Aleppo, east of Idlib, on Friday night, forcing the army to redeploy.

In remarks published on state media, Assad said: "Terrorists only know the language of force and it is the language we will crush them with."

The Syrian army said dozens of its soldiers had been killed in the fighting in Aleppo.

Russian war bloggers reported on Sunday that Moscow had dismissed Sergei Kisel, the general in charge of its forces in Syria. Reuters has requested comment from the Russian defence ministry.

Civilians leaving Aleppo

Inside Aleppo city, streets were mostly empty and many shops were closed on Sunday as scared residents stayed at home. There was still a heavy flow of civilians leaving the city, witnesses and residents said.

Rebel fighters waving the opposition flag drove through the city, Yusuf Khatib, a resident, told Reuters by phone. Some rebels took up positions on street intersections, he added.

Ahmad Tutenji, a merchant in the affluent New Aleppo neighbourhood, said he was surprised how quickly the army left. "I am shocked at how they fled and abandoned us."

Syrian troops who had withdrawn from the city were now regrouping and reinforcements were also being sent to help in the counter-attack, army sources said.

Aleppo had been firmly held by the government since a 2016 victory there, one of the war's major turning points, when Russian-backed Syrian forces besieged and laid waste to rebel-held eastern areas of what had been the country's largest city.

A lack of that manpower contributed to the speedy retreat of Syrian army forces in recent days, according to two army sources.

The rebel gains came after Israel stepped up its strikes on Iranian bases in Syria and Iran-backed Hezbollah forces in Lebanon. Militias allied to Iran, led by Hezbollah, have had a strong presence in the Aleppo area.

The head of the U.S.-backed Syrian Democratic Forces (SDF) said in a statement on Monday that its fighters in northwestern Syria were facing intense attacks on multiple fronts.

SDF commander Mazloum Abdi said the group had tried to establish a "humanitarian corridor" linking Kurdish-held northeastern regions to Tel Rifaat, a strategic area northwest of Aleppo to "protect our people from potential massacres."

"However, attacks by armed groups supported by the Turkish occupation have disrupted this corridor," Abdi said, adding that "our forces continue to resist to protect our people in the Kurdish neighborhoods of Aleppo."

Turkey considers Kurdish militant groups the Kurdistan Workers Party (PKK) and the Syrian Kurdish YPG as terrorist organizations, as well as Hayat Tahrir al-Sham.

Syrian rebels backed by Turkey had blocked an attempt by Kurdish groups to establish a corridor connecting Tel Rifaat to northeastern Syria, Turkish security sources said on Sunday.

Turkish Foreign Minister Hakan Fidan told U.S. Secretary of State Antony Blinken on Sunday that Turkey would not allow what he called terrorist threats targeting its security or Syrian civilians, Turkish diplomatic sources said.

The war, which has killed hundreds of thousands of people and displaced many millions, has ground on since 2011 with no formal end. Most heavy fighting halted years ago after Iran-backed militias and Russian air power helped Assad win control of all major cities.

Iranian Foreign Minister Abbas Araqchi, visiting Damascus on Sunday, said the situation in Syria was "difficult" but the Assad government would prevail.