The Syrian army said on Saturday dozens of its soldiers had been killed in a major attack led by rebels who swept into the city of Aleppo, forcing the army to redeploy in the biggest challenge to President Bashar al-Assad in years.
Russia's Defence Ministry said its air force had carried out strikes on Syrian rebels in support of the country's army, Russian news agencies reported. The strikes followed what was the boldest rebel assault for years in a civil war where front lines had largely been frozen since 2020.
The rebel group, Hayat Tahrir al-Sham, or HTS, once known as the Nusra Front, is designated a terrorist group by the US, Russia, Turkey and other states. Assad is a close Moscow ally.
In Washington, the White House National Security Council said it was closely monitoring the situation and had been in contact with regional capitals over the last 48 hours.
NSC spokesperson Sean Savett said Syria's refusal to engage in a political process and its reliance on Russia and Iran had "created the conditions now unfolding, including the collapse of Assad regime lines in northwest Syria."
Savett said the US had nothing to do with the offensive led by "a designated terrorist organisation" and "urged de-escalation...and a serious and credible political process" under the 2015 UN Security Council Resolution 2254, which laid out the steps for a ceasefire and political transition.
The war, which has killed hundreds of thousands of people and displaced many millions, has ground on since 2011 with no formal end, although most major fighting halted years ago after Iran and Russia helped Assad's government win control of most land and all major cities.
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.
Acknowledging the rebel advance, the Syrian army command said insurgents had entered much of Aleppo.
After the army said it was preparing a counterattack, airstrikes targeted rebel gatherings and convoys in the city, the pro-Damascus newspaper al-Watan reported. One strike caused casualties in Aleppo's Basel square, a resident told Reuters.
The state-run Russian Centre for the Reconciliation of the Enemy Parties in Syria said missile and bomb strikes against the rebels had targeted "militant concentrations, command posts, depots, and artillery positions" in Aleppo and Idlib provinces. It claimed about 300 rebel fighters had been killed.
Images filmed on Saturday showed people posing for photos on a toppled statue of Bassil al-Assad, late brother of the president. Fighters zipped around the city in trucks and milled around in the streets. A man waved a Syrian opposition flag as he stood near Aleppo's historic citadel.
The Syrian military command said rebels had attacked in large numbers and from multiple directions, prompting "our armed forces to carry out a redeployment operation aimed at strengthening the defence lines in order to absorb the attack, preserve the lives of civilians and soldiers".
The rebels also took control of Aleppo airport, according to a statement by their operations room and a security source.
Two rebel sources also said the insurgents had captured the city of Maraat al Numan in Idlib province, bringing all of that area under their control.