By Eyad Kourdi, Gul Tuysuz, Sophie Tanno, Tim Lister and Mohammed Tawfeeq, CNN

(CNN) — Syrian opposition forces have taken control of much of the country’s second-largest city Aleppo after a lightning advance that killed dozens of government soldiers in a major challenge to President Bashar al-Assad.

A rebel alliance launched a surprise attack this week, sweeping eastward through villages outside the city and reigniting a conflict that had been largely static for years. It is the first time Syrian rebels have set foot in Aleppo since government forces regained control during the civil war in 2016.

By Saturday morning, rebel fighters had taken control of large swathes of the city, according to footage geolocated by CNN.

The Syrian army tacitly acknowledged its forces were in retreat, saying “large numbers of terrorists” had forced it to “implement a redeployment operation.” It said reinforcements were on the way and government forces were preparing for a “counteroffensive.”

Rebel fighters have been seen at key locations, with one video showing armed men waving an opposition flag and yelling “God is great” in Arabic at a central square.

Another clip shows rebels at the city’s citadel, which is also in central Aleppo. At least one man in the clip is armed, as he says: “We are the first to arrive and the first to conquer.”

The rebels also claim to have taken the city’s airport, about 4.5 miles (7.2 kilometers) east of the city center. Imagery captured inside the airport and verified by CNN shows camo-clad fighters inside the main terminal.

The only exception appears to be the northeastern part of the city, where a few neighborhoods remain under the control of government forces and Iranian militia allies.

The rebel forces have declared a 24-hour curfew to begin at 5 p.m. local time Saturday, which they said was to ensure “the safety of the residents of the city and to secure private and public property from tampering or harm.”

Syria’s defense ministry said dozens of soldiers have been killed in the Aleppo offensive.

It appears the advancing rebels met little resistance from the Syrian army, with several residents inside Aleppo telling CNN that there has been minimal fighting in the city’s urban areas.

In response to the rebel advance, the Russian air force launched an aerial offensive against Syrian armed opposition forces in Aleppo and Idlib provinces, Russian state media reported. On Saturday, the Syrian Defense Ministry confirmed that “Russian and Syrian warplanes” had carried out strikes targeting what it claimed were “terrorist gatherings, movements and supply lines” in and around Aleppo.

Activists and journalists in Aleppo and Idlib told CNN that Russian warplanes had carried out at least 10 strikes on rebel positions in the countryside of Aleppo and Idlib since Wednesday.

A Syrian airstrike near al Basil roundabout in Aleppo on Saturday also resulted in casualties, journalists told CNN, citing health workers in the city.

CNN cannot independently confirm the specific attacks or casualties.

Video from the west of the city showed multiple casualties after an airstrike Saturday. It’s not known whether a Syrian or Russian aircraft carried out the strike.

The video showed at least seven bodies as well as people with severe burns.

Kurdish forces have also expanded their control of some neighborhoods of Aleppo, residents said. Prior to this week’s attack they held two Kurdish neighborhoods but have now moved into areas that the Syrian regime used to control.

The Kurdish militia, known as the YPG, has a history of conflict with other rebel groups in northern Syria.

There has already been one clash Saturday between members of the rebel alliance and Kurdish fighters inside the city, according to video analyzed by CNN. Part of the rebel coalition says it now intends to launch an offensive against the Kurdish groups holding parts of northern Aleppo province.

Losing Aleppo marks a significant setback for Assad’s forces. Once Syria’s largest city by population and its economic capital, it is one of the oldest inhabited cities in the world.

Aleppo was also the main rebel stronghold until Assad took it over in 2016. With the rebels regaining a foothold there again, they are no longer cornered in Idlib, which could potentially trigger a domino effect.

Rebels lay out their goals

The rebels are part of a newly formed coalition called the “Military Operations Command,” which includes a broad spectrum of opposition fighters including Islamist factions and moderate groups once backed by the US.

The coalition was announced Wednesday ahead of the attack on Aleppo and said it was responding to escalating attacks from the Syrian government and Iranian militias. But the timing is also critical, with Syria’s key backer Russia focusing on Ukraine and its other major ally, Iran, on the back foot from Israeli attacks on it and its proxy network.

The offensive is the first major flare-up in years between the Syrian opposition and the regime of President Assad, who has ruled the war-torn country since 2000.

Syria’s civil war began during the 2011 Arab Spring as the regime suppressed a pro-democracy uprising against Assad. The country plunged into a full-scale civil war as a rebel force was formed, known as the Free Syrian Army, to combat government troops.

Since the 2020 ceasefire agreement, the conflict has remained largely dormant, with low-level clashes between the rebels and Assad’s regime.

More than 300,000 civilians have been killed in more than a decade of war, according to the United Nations, and millions of people have been displaced across the region.