Syrian and Russian jets are stepping up strikes on opposition forces in northern Syria in retaliation for the sudden offensive that has cost the regime control of the country’s second largest city, Aleppo.

The offensive has also led to the capture by the rebel alliance of an important military base east of Aleppo and large areas of both Aleppo and Idlib provinces. It has met little resistance on the ground from regime forces and also comes at a time when Syria’s key backers – Iran and Russia – are focusing on their own conflicts.

The rebels’ sweeping success has posed the biggest challenge in eight years to President Bashar al-Assad, when Russian air power helped reverse rebel gains in the civil war.

The newly formed rebel coalition, which calls itself the Military Operations Command, has captured key sites across Aleppo, including the airport, where video verified by CNN showed camo-clad fighters inside the main terminal.

The rebels consolidated their gains Sunday capturing key military sites in the east of Aleppo city. But they have left some neighborhoods in the hands of Kurdish forces.

The opposition forces’ control of Aleppo means that the regime counteroffensive promised by the Syrian defense ministry would be very difficult to carry out.

Government aircraft – along with Russian planes based in Syria – have carried out bombing raids against opposition positions in Aleppo, Idlib and Hama.

The official Russian news agency TASS cited the Syrian army command in a report Sunday as saying that their air forces “stepped up strikes on terrorist positions and their supply lines, with dozens killed and wounded.”

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

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

Meanwhile, a Russian strike damaged a Franciscan convent in Aleppo, according to the Italian Foreign Minister Antonio Tajani, though Vatican News said no injuries had been reported.

The White Helmets, a Syrian volunteer service, said at least four people were killed on Sunday in airstrikes on Idlib city, a province that now appears to be entirely in rebel hands.

In his first comments since the lightning takeover, Assad has said that Syria will continue “to defend its stability and territorial integrity in the face of all terrorists and their supporters,” during calls with regional leaders on Saturday.

Assad said Syria was capable “with the help of its allies and friends, of defeating and eliminating them, no matter how intense their terrorist attacks are.”

The rebel offensive has reignited Syria’s long-running civil war, which killed more than 300,000 people and created nearly 6 million refugees. The conflict never formally ended and the flare-up is the most significant since 2020, when Russia and Turkey reached a ceasefire in Idlib.

On Sunday, Assad told Abbas Araghchi – the visiting foreign minister of Iran, which has backed Assad in the civil war – that he intends to fight “with all force and determination and throughout (Syria’s) territory.”

The rebels are led by Hayat Tahrir al-Sham (HTS), a former al Qaeda affiliate in Syria that used to go by the name Al-Nusra Front, along with groups backed by Turkey and others previously supported by the US.

This presents a dilemma for Western governments, Asli Aydintasbas, a Visiting Fellow at Brookings Institution, told CNN.

“Should they be cheering the opposition taking over Syria’s second-largest city Aleppo, or should they actually worry about the city falling under Islamist rule?” she said.

Aydintasbas believes the events that have unfolded in Syria show a new balance of power in the country, with Turkey emerging as a “major actor,” while Russia’s power is weakened and Iran is “on its back foot.”

Complicating matters is that some members of the rebel coalition are also fighting Kurdish forces.

The Turkish-backed Free Syrian Army, which is part of the rebel coalition, said Sunday it had seized control of the city of Tal Rifaat and the towns of Ain Daqna and Sheikh Issa in the northern part of the Aleppo governorate. It also claimed to have captured the villages of Shaaleh and Nairabiyyeh in Aleppo’s northern countryside.

Those territories were previously held not by the government of Bashar al-Assad but by another faction involved in the multi-front civil war, the Syrian Democratic Forces.

The Syrian Democratic Forces are largely made up of Kurdish fighters from a group known as the Peoples’ Protection Units (YPG), which is considered a terrorist organization by neighboring Turkey.

The Syrian Democratic Forces have previously fought other Syrian opposition groups, but have in the past received backing from the US for its fight against ISIS.