Rebel forces in Syria have seized control of most of the city Aleppo after launching a surprise offensive against government forces, as Russia says it deployed fighter jets against the rebels.
An alliance of rebel factions led by the Islamist militant group Hayat Tahrir al-Sham (HTS) has advanced deep into Aleppo and now controls the majority of the northern city, the Syrian Observatory for Human Rights said.
The insurgence, which began on Wednesday, appears to have taken President Bashar al-Assad's regime by surprise, with experts describing it as a significant turning point.
Thousands of fighters from the rebel groups made major territorial gains within a few days in the area around Idlib and Aleppo, the second-largest city which had been under government control for years.
The alliance now appears set on taking control of the entire neighbouring province of Idlib. On Friday, the rebels advanced to the western outskirts of Aleppo and from there continued to enter the city.
The future course of events will largely depend on decisions made in Russia, one of al-Assad's closest allies.
Russia says it deployed fighter jets in Syria
Russia announced on Saturday that its fighter jets had conducted multiple missions in Syria, targeting rebel command posts, artillery positions, and camps, reportedly killing around 300 fighters.
Captain Oleg Ignasyuk, head of the Russian mission in Syria, said that operations against "extremist aggression" would continue, the state-run news agency TASS quoted him as saying.
The information could not be independently verified. Ignasyuk did not provide information about the locations of the fighter aircraft.
Russia, a key ally of the Syrian government, has supported President Bashar al-Assad since 2015 and stationed forces at the Khmeimim airfield and the port city of Tartus.
The Syrian military said the large number of insurgents involved in the surprise multi-front offensive prompted government forces to redeploy and prepare for a counter-attack.
The rebels' offensive marks a significant escalation in Syria's civil war, which has raged since 2011 and had recently seen relatively stagnant front lines.
US says it has nothing to do with rebel offensive
The US, meanwhile, has distanced itself from the ongoing rebel offensive.
"The United States has nothing to do with this offensive, which is led by Hay'at Tahrir al-Sham (HTS), a designated terrorist organisation," White House National Security Council spokesman Sean Savett said in a statement released late on Saturday.
Savett blamed al-Assad's reliance on Russia and Iran, along with his refusal to pursue a political solution, for creating the "conditions now unfolding."
Savett also called for "de-escalation, protection of civilians and minority groups, and a serious and credible political process" to end the civil war, in line with UN Security Council Resolution 2254. Adopted in 2015, the resolution calls for peace talks between the Syrian government and opposition forces.
A few hours after the Syrian government confirmed that it had lost check of large parts of Aleppo to rebels, the centre was hit by a fierce airstrike.
At least 16 people were killed, according to the observatory. The suspected attack by Russian fighter jets on Saturday afternoon killed at least 16 people and injured 20 others, the observatory said.
The current fighting has killed at least 327 people and displaced at least 50,000 others, the war monitor estimated.
Mustapha Bakour, spokesman for Jaish al-Izza, a faction participating in the rebel attack on Aleppo, said the opposition fighters are currently in the final stages of combing through the city.
He told dpa that the advance had been swift because the rebels prepared well for the attack and Iran-backed militias loyal to the Syrian government had withdrawn.
HTS is considered one of the most powerful armed militias in north-western Syria.
The opposition assault is considered the largest by rebels on Aleppo since 2016 when they were ousted from the eastern areas of the city.
Iran, Syria's most important ally after Russia, has significantly expanded its military influence in recent years, establishing a land corridor through Iraq and Syria to Lebanon.
Syria remains a critical part of Iran's Middle East strategy. Idlib, south-west of Aleppo, is the last rebel stronghold in war-torn Syria.
On Saturday, Syrian rebels reportedly captured at least 11 villages in the countryside around Hama, where Syrian government forces were stationed, the war monitor said.
"What we are witnessing in Syria now is a total collapse of government troops," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told dpa.
According to one expert, the success of the collective offensive by the Syrian rebel groups on Aleppo depends largely on Moscow's reaction.
