Syrian and Russian warplanes are stepping up attacks on rebels as the opposition captures large parts of Aleppo

Syrian and Russian fighter jets are targeting opposition fighters in northern Syria in response to the regime’s loss of control of Aleppo, the country’s second-largest city.

During the attack, the rebel alliance also took control of a major military facility east of Aleppo as well as large parts of Aleppo and Idlib provinces. Resistance on the ground from regime forces is minimal and comes at a time when Syria’s main backers, Iran and Russia, are busy fighting.

The rebel victory presented President Bashar al-Assad with the biggest challenge since the Russian air force reversed the insurgents’ gains in the civil war eight years ago.

The newly formed rebel coalition, known as the Military Operations Command, has seized large areas across Aleppo, including the airport, where videos confirmed by CNN show camouflaged militants in the main terminal.

The rebels consolidated their gains on Sunday and captured key military sites in eastern Aleppo. However, they have left several neighborhoods under the control of Kurdish forces.

Due to opposition forces’ control of Aleppo, the regime’s counteroffensive promised by the Syrian Defense Ministry will be extremely difficult to implement.

Government aircraft have carried out bombing raids on opposition positions in Aleppo and Idlib provinces in cooperation with Russian planes based in Syria.

According to the official Russian news agency TASS, the Syrian army leadership said on Sunday that its air forces had “intensified attacks on terrorist positions and supply lines, killing and wounding dozens.”

An airstrike near Aleppo University killed at least four people on Sunday, according to a social media video geolocated by CNN.

It is unclear whether the attack was carried out by Russian or Syrian regime aircraft. The strike follows a strike on Saturday in which several people were killed in a square in western Aleppo.

At least four people were killed Sunday in airstrikes on the city of Idlib, a province that now appears to be fully under opposition control, according to the White Helmets, a Syrian volunteer organization.

In talks with regional leaders on Saturday, Assad said Syria would continue to “defend its stability and territorial integrity in the face of all terrorists and their supporters,” in his first comments since quickly seizing power.