A war monitor, the Syrian Observatory for Human Rights, said the insurgents led by Salafi Islamist militant group Hayat Tahrir al-Sham, – including Al-Qaeda's former Syria branch, Jabhat al-Nusra – and allied factions seized control of Aleppo International airport, the first international airport to be controlled by militants. They allegedly seized the airport and posted pictures from there.

Thousands of militants also moved on, facing almost no opposition from government forces, to seize towns and villages in northern Hama, a province where they had a presence before being expelled by government troops in 2016. They claimed Saturday evening to have entered the city of Hama.

The swift and surprise offensive raises questions about Syria's armed forces' preparedness. The insurgent offensive launched from their stronghold in the country's northwest appeared to have been planned for years. It also comes at a time when Syria's allies were preoccupied with their own conflicts.

In his first public comments since the start of the offensive, released by the state news agency Saturday evening, Syrian president Bashar Al-Assad said Syria will continue to "defend its stability and territorial integrity against terrorists and their supporters.” He added that Syria is able to defeat them no matter how much their attacks intensify.

Turkey, a main backer of Syrian opposition groups, said its diplomatic efforts had failed to stop government attacks on opposition-held areas in recent weeks, which were allegedly in violation of a de-escalation agreement sponsored by Russia, Iran and Ankara.

Turkish security officials alleged a limited offensive by the militants was planned to stop government attacks and allow civilians to return, but the offensive expanded as Syrian government forces began to retreat from their positions.

The insurgent launched their shock offensive on Wednesday. They first staged a two-pronged attack in Aleppo and the Idlib countryside, entering Aleppo two days later and securing a strategic town that lies on the highway that links Syria's largest city to the capital and the coast.

By Saturday evening, they seized at least four towns in the central Hama province and claimed to have entered the provincial capital. The insurgents staged an attempt to reclaim areas they controlled in Hama in 2017 but failed.

Preparing a counterattack
 

Syria’s armed forces said in a statement Saturday that to absorb the large attack on Aleppo and save lives, it redeployed troops and equipment and was preparing a counterattack. The statement acknowledged that insurgents entered large parts of the city but said they had not established bases or checkpoints.

Later on Saturday, the armed forces sought to dispel what it said were lies in reference to reports about its forces retreating or defecting, saying the general command was carrying out its duties in “combatting terrorist organizations.”

The return of the militants to Aleppo was their first since 2016, following a military campaign in which Syria's armed forces were backed by Russia, Iran and allied groups.

The 2016 battle for Aleppo was a turning point in the war between Syrian government forces and opposition militants after 2011 protests against Assad’s rule turned into an all-out war. After appearing to be losing control of the country to the militants, the Aleppo battle secured Assad’s hold on strategic areas of Syria, with opposition factions and their foreign backers controlling areas on the periphery.

The lightning offensive threatened to reignite the country's civil war, which had been largely in a stalemate for years.

Late on Friday, witnesses said two airstrikes hit the edge of Aleppo city, targeting insurgents reinforcements and falling near residential areas. The Observatory said 20 insurgents were killed.

Militants were filmed outside police headquarters, in the city center, and outside the Aleppo citadel, the medieval palace in the old city center, and one of the largest in the world. They tore down posters of Assad, stepping on some and burning others.

The push into Aleppo followed weeks of simmering low-level violence, including government attacks on opposition-held areas.

City's hospitals are full
 

Aleppo residents reported hearing clashes and gunfire, but most stayed indoors. Some fled the fighting.

Schools and government offices were closed Saturday as most people stayed indoors, according to Sham FM radio, a Syrian station. Bakeries were open.

The U.N. Office for the Coordination of Humanitarian Affairs said Friday Aleppo's two key public hospitals were reportedly full of patients while many private facilities closed.

In social media posts, the militants were pictured outside of the citadel, the medieval palace in the old city center, and one of the largest in the world.

The Syrian Kurdish-led administration in the country's east said nearly 3,000 people, most of them students, had arrived in their region after fleeing the fighting in Aleppo, which has a sizeable Kurdish population.

State media reported that a number of “terrorists," including sleeper cells, infiltrated parts of the city. Government troops chased them and arrested a number who posed for pictures near city landmarks, they said.

On a state TV morning show Saturday, commentators said army reinforcements and Russia’s assistance would repel the “terrorist groups,” blaming Turkey for supporting the insurgents’ push into Aleppo and Idlib provinces.

Russia’s state news agency Tass quoted Oleg Ignasyuk, a Russian Defense Ministry official coordinating in Syria, as saying that Russian warplanes targeted and killed 200 insurgents who had launched the offensive in the northwest on Friday.