Islamist rebels once exiled to a mountainous pocket of the Syrian countryside now roam the streets of central Aleppo, taking pictures below its ancient citadel and tearing down symbols of President Bashar al-Assad’s rule.

The surprise offensive in which insurgents seized territory across north-western Syria appears to have dramatically shifted the balance of power in Aleppo, the country’s second largest city, and marks the most serious challenge to Assad’s control in years.

On Saturday night, images emerged of fighters pushing deep into Syrian government-controlled territory towards the city of Hama, including Kafr Nabl, a town once seen as symbolic for its opposition to Assad.

Within hours, video showed insurgents in the centre of Hama. Syria’s state news agency, Sana, quoted military sources denying reports of a rebel advance in Hama, adding “our armed forces have a reinforced defensive line”.

Sana said government forces were repelling insurgent advances with the help of Russian aircraft, while opposition networks reported air strikes in the Idlib countryside.

Fighters from the militant Islamist group Hayat Tahrir al-Sham (HTS) seized much of Aleppo less than a day earlier in a sudden rout of Syrian army forces. A reporter with the opposition television channel Aleppo Today showed uniformed militants in an empty central plaza.

Footage showed people tearing down a statue of Bassel al-Assad, the brother of Syria’s ruler, to the sound of celebratory gunfire. Turkey’s Anadolu news agency said Syrian forces withdrew from several key locations including the civilian airport, closing it as insurgents closed in.

Forces spearheaded by HTS also seized an important military base to the south while taking control of Saraqib, a strategic location on the highway to the capital Damascus.

Turkish-backed Syrian rebels launched their own operation against Kurdish militants and Syrian government forces in an effort to seize a military airport to the east of Aleppo, as swaths of territory rapidly fell under rebel control.

The sweeping offensive appeared to surprise forces loyal to Assad as well as his longtime backers in Moscow and Tehran.

In a late-night phone call with the Emirati president, Sheikh Mohamed bin Zayed al-Nahyan, Assad stressed that Syria “will continue to defend its stability and territorial integrity”. He added that Damascus is capable of beating back the advance “with the help of its allies”.

The Iranian foreign minister, Abbas Araghchi, is expected in Damascus on Sunday before travelling to Turkey, while Russian foreign minister Sergei Lavrov discussed the situation in Syria with his Turkish counterpart Hakkan Fidan in a phone call.

Iran’s Tasnim news agency said the Syrian military continued to battle insurgents in Aleppo, amid reports of both Russian and Syrian airstrikes around the city.

The militants appeared to enter Aleppo with ease, in total contrast to the fierce street battles for control of every block that engulfed the ancient urban centre 12 years ago. “No one expected Aleppo to be taken, which means there were no real defensive lines within the city. Once they got there it seems like it was all open,” said Jerome Drevon of the International Crisis Group.

Drevon pointed to the insurgents’ years-long efforts to formalise and hone their forces, allowing them to overwhelm far less organised Syrian government fighters. “I think the regime didn’t expect such a quick move, the operation started just a few days ago,” he said.

The Syrian military said the overwhelming number of fighters “and the multiplicity of battlefronts prompted our armed forces to carry out a redeployment operation aimed at strengthening the defence lines in order to absorb the attack, preserve the lives of civilians and soldiers, and prepare for a counterattack.”

Dmitry Peskov, a Kremlin spokesman, called the situation in Aleppo “an attack on Syrian sovereignty”, adding: “We are in favour of the Syrian authorities bringing order to the area.”

What began in 2011 as a popular uprising calling for Assad’s overthrow later transformed into a bloody civil war, with the battle for control of Aleppo at its heart. Syrian regime forces seized control of the city in 2016, with the aid of Russian air power and Iranian ground forces. As he fought for control of the country, Assad also freed jihadi fighters from the country’s prisons, transforming the uprising against him.

The sudden insurgent victory in Aleppo symbolised a dramatic shift in control of key urban centres in Syria, and an unexpected challenge to its president, who had long been seen as having crushed the uprising.

Assad’s fractured control of the country had appeared secure enough that his former regional foes, notably Saudi Arabia, had begun to re-establish diplomatic relations with Damascus.

Turkish officials, who had also discussed normalising relations with Assad, despite supporting rebel forces, denied any involvement in the Aleppo offensive. “We will not take any action that could cause a wave of migration,” the foreign minister, Fidan, said amid reports from the UN that fighting had internally displaced 14,000 people in days.

The insurgents’ sudden success quickly drew questions about their ability to hold territory, and what an expanded fiefdom led by HTS’s leader known as Abu Mohammad al-Jolani could look like. Jolani was designated by the US State Department as a terrorist in 2013 and retains a $10m bounty on his head, but has de facto ruled Idlib province for several years.

While the militants in Idlib have attempted to demonstrate their ability to govern, they have also stood accused of crushing dissent while relying heavily on dwindling international aid to meet civilians’ needs. As fighters stormed Aleppo, humanitarians like Sudipta Kumar of Actionaid warned many were suffering in Idlib.

“Thousands of families now face a freezing winter without anywhere to live,” she said.

Sam Heller, an analyst at the Century Foundation, said the insurgents’ ability to hold on to their territorial gains depended on whether Damascus and its allies were able to mount a counter-attack.

“Certainly some areas in the Aleppo countryside could be difficult for HTS and their allies to hold on to if they come under really withering air strikes or artillery fire,” he said. Insurgent rule inside Aleppo itself, he added, could prove far more difficult for Assad and his allies to repel in the long term.

“It’s not clear what kind of capabilities Damascus is now able to bring to bear and mobilise from elsewhere in Syria, also critically how much capacity Russia now has in Syria, given its current involvement in Ukraine which has diverted some of their forces to that front.”

Drevon doubted that Jolani and his allies would be willing to cede power to a conventional governing authority. The militants were more likely to focus on expanding the field of battle for now, he said, adding: “They have been waiting for this battle for a long time.”