Syrian and Russian jets have launched air strikes on Syrian rebels who are advancing through the country after seizing its second-largest city.
The rebels, led by jihadi group Hayat Tahrir al Sham, took over most of Aleppo in a shock offensive on Saturday. They claim to have also entered the city of Hama.
Air strikes that hit the entrance of Aleppo University Hospital killed 12 civilians and injured 23 others, said the Syrian Civil Defence, also known as the White Helmets.
The Syrian army and Russia denied attacking civilians and said they had targeted insurgent hideouts.
Images from the city of Idlib also showed the aftermath of a strike with a vehicle on fire and debris covering the street.
A White Helmets volunteer told Sky News: "War planes are everywhere, targeting everything.
"The situation is complex but the only thing which is true is that the civilians [have] paid the price from 2011 up until now. And now, a new episode is happening."
The Syrian regime has also deployed military personnel with rocket launchers, its defence ministry said on Facebook.
Russia has long been a key ally for Syria's President Bashar al Assad and helped him regain control in 2016 following an uprising that began in 2011.
The air strikes come after Mr Assad said in his first public comments that his country would "defend its stability and territorial integrity" and defeat the "terrorists and their supporters".
On Saturday, thousands of insurgents - many of whom had previously been forced into retreat in Idlib Governate - took over most of Aleppo and faced little to no resistance from government troops, according to fighters and activists.
They also seized towns and villages in northern Hama, a province where they had a presence before being expelled in 2016.
The swift offensive is a huge embarrassment for Mr Assad and raises questions about his forces' preparedness.
The insurgents, including Turkey-backed fighters, began the attacks on Wednesday and entered Aleppo two days later.
At least 327 people, including 44 civilians, have been killed since, according to the UK-based Syrian Observatory for Human Rights (SOHR). Thousands have also reportedly been displaced.
Russian war bloggers claimed on Sunday that the general in charge in Syria, Sergei Kisel, had been sacked and replaced.
However, the move has not yet been officially confirmed.
Russia's defence ministry first said its air force had carried out strikes on Saturday, Russian news agencies reported.
The state-run Russian Centre for the Reconciliation of the Enemy Parties in Syria said attacks had targeted "militant concentrations, command posts, depots, and artillery positions" in Aleppo and Idlib provinces.
It claimed about 300 rebel fighters had been killed.
The strikes follow the boldest rebel assault for years in a civil war where front lines had largely been frozen since 2020.
The long-simmering war, which has killed hundreds of thousands and displaced millions, has ground on for more than a decade with no formal end in sight.
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As well as Russia, Mr Assad's allies include Iran whose foreign minister, Abbas Araqchi, was in the Syrian capital Damascus on Sunday.
According to state media, he described the situation as "difficult" but said the Syrian government would prevail.
Arab leaders including Jordan's king and the president of the UAE have also expressed solidarity with the Syrian government in calls with Mr Assad.
Meanwhile, the White House said it was "closely monitoring the situation" but "has nothing to do with this offensive".
A statement added: "The United States, together with its partners and allies, urge de-escalation, protection of civilians and minority groups, and a serious and credible political process that can end this civil war once and for all with a political settlement consistent with UNSCR 2254."
The United Nations Security Council Resolution 2254 was unanimously adopted in December 2015 - calling for a ceasefire and political settlement in Syria. However, no real progress has been made to implement the resolution.
The US maintains a presence in parts of eastern Syria, as part of coalition efforts to restrict the operations of Islamic State (ISIS) militants.
(c) Sky News 2024: Russian jets strike Syria after rebels seize Aleppo in shock offensive