Syria's army admitted that rebels had entered "large parts" of Aleppo and said "dozens of men from our armed forces were killed and others wounded".
The fighters have pressed a lightning offensive against forces of the Iranian- and Russian-backed Syrian government since Wednesday, the same day a fragile ceasefire took effect in neighbouring Lebanon between Israel and the Iran-backed Hezbollah group, a Damascus ally, after two months of all-out war.
"Hayat Tahrir al-Sham (HTS) and allied factions... took control of most of the city and government centres and prisons", said the Britain-based Syrian Observatory for Human Rights.
HTS, a jihadist alliance led by Al-Qaeda's former Syria branch, controls swathes of the Idlib region in Syria's northwest, as well as parts of neighbouring Aleppo, Hama and Latakia provinces. Allied Turkish-backed rebel factions have also been taking part in the offensive.
Overnight, Russian "warplanes launched raids on areas of Aleppo city for the first time since 2016", added the Observatory which relies on a network of sources inside Syria.
AFPTV images showed fires burning in part of the city after the air strikes. Streets were largely deserted but AFP images showed rebels outside a Syrian Telecom building and the landmark citadel.
An opposition flag hung from a traffic light.
Praying in the city
Some rebels prayed in the city centre and fired celebratory gunfire, later patrolling on foot in central Aleppo, images showed.
Other fighters were seen posing with a rebel flag outside a police building bearing a large poster of Assad, and standing below a partially-burning billboard of the president.
Syria's army said "armed terrorist organisations" -- a term it also uses to refer to opposition groups -- launched "a broad attack from multiple axes on the Aleppo and Idlib fronts" and reported fierce battles "over a strip exceeding 100 kilometres (60 miles)".
The Observatory said the overnight air strikes coincided with "the arrival of large (rebel) military reinforcements" to the area. The war monitor also reported that the rebels had taken dozens of towns and villages in the north.
It raised the death toll in days of clashes to 311 -- 183 from HTS and allied factions, 100 soldiers and pro-government forces, as well as 28 civilians.
State media reported that four civilians were killed when HTS shelled a student residence in Aleppo, a city of around two million people that was Syria's pre-war manufacturing hub.
Pro-government radio station Sham FM reported that "armed groups were present in a number of streets and neighbourhoods in Aleppo city amid air strikes targeting fighters' positions west of the city".
"Most civilians are avoiding leaving their homes and public and private institutions in the city are almost completely shut," it added.
Iran-backed groups have a heavy presence in the Aleppo region after providing crucial ground support when the Syrian army -- backed by Russian air power -- recaptured rebel-held areas of the city in 2016.
The Observatory said that "the governor of Aleppo and the police and security branch commanders withdrew from the city centre".
Observatory director Rami Abdel Rahman told AFP early Saturday that rebel fighters swiftly captured swathes of Aleppo without meeting significant resistance, adding "there has been no fighting, not a single shot was fired, as regime forces withdrew."
The Russian military said Friday it was bombing "extremist" forces, as Turkey demanded a halt to bombardment on the Idlib region.
Since 2020, the Idlib area has been subject to a Turkey and Russia-brokered truce which had largely been holding despite repeated violations.
State news agency SANA said Saturday that Foreign Minister Bassam al-Sabbagh discussed "the latest developments in northern Syria" with his Egyptian counterpart Badr Abdelatty.
