Rebel forces have swept into Syria’s second city Aleppo after mounting a lightning offensive that poses the biggest threat in years to Bashar al-Assad’s regime. 

The Syrian army said on Saturday that the rebels had been able “to enter wide areas of Aleppo city but were unable to secure strongholds because of continued powerful and targeted strikes by our armed force”.

It added that it was preparing for a counterattack and that its forces had engaged in “fierce battles” in an area spanning 100km in recent days.

The rebels, led by Islamist group Hayat Tahrir al-Sham, said on Saturday that its fighters had advanced in multiple directions from their stronghold in Idlib province in northwestern Syria and had taken control of several dozen towns and a regime air base.

Images circulated on opposition-linked social media showed rebel forces, who launched their offensive on Wednesday, posing in front of Aleppo’s citadel, which lies in the heart of the city.

The assault comes as Assad faces growing domestic and external pressures in a country shattered by civil war that erupted after a 2011 popular uprising. He was able to quash the original rebellion with military backing from Russia, Iran and Iranian-backed groups, including Hizbollah, the Lebanese militant movement.

The fighting in Syria’s civil war had largely diminished in recent years, with the remnants of the armed opposition pushed to northern and northwestern areas of the country close to the Turkish border.

But over the past year, Israel has stepped up its air strikes on Iranian-affiliated targets in Syria as it has launched an offensive against Hizbollah in Lebanon, weakening the groups that had played a crucial role in supporting the Assad regime. The Israeli military said it struck “military infrastructure” linked to Hizbollah in Syria near the Lebanese border on Saturday.

HTS’s ability to fight inside Aleppo is a devastating blow to Assad and underscores the regime’s weakness.

“This is very serious for Assad,” said Malik al-Abdeh, a Syrian analyst. “Israel’s attacks against Iran and Hizbollah created the window of opportunity for this to happen. The long attritional war between Israel and Iran has clearly taken its toll on Iran’s capacity to deploy and fight in Syria.” 

He added that HTS had been planning the assault for months and was co-ordinating with Turkish-backed factions, known as the Syrian National Army, although the latter had yet to deploy in full force.

“People in the regime areas have become so demoralised, they have no hope and will welcome any challenge to the Syrian regime,” Abdeh said. “And the Syrian army is no longer prepared to die for the regime any more.” 

The Syrian military said that dozens of regime forces had been killed in the fighting. It added that the scale of the rebel offensive had forced the military to carry out a temporary “redeployment operation” whose goal was to shore up defences and allow it to prepare a counter attack.

The Syrian Observatory for Human Rights, a UK-based monitoring group, said that HTS had taken control of more than half of the city of Aleppo in just a few hours “without any resistance from regime forces”. 

The fighting has displaced large numbers of civilians in Aleppo and the surrounding countryside, the UN and Syrian Observatory for Human Rights said.

Aleppo was the scene of some of the heaviest fighting of the civil war. After laying siege to the city with the support relentless Russian bombing, it drove out rebels based in Aleppo’s eastern neighbourhoods. That turned the war in Assad’s favour.

Emile Hokayem, at the International Institute for Strategic Studies, said the Assad regime “still has manpower, air power and external support”.

“But the loss of Aleppo is a monumental loss that will shake the confidence of regime loyalists,” Hokayem said.

“Assad thought he was back in the geopolitical game because of the desire of other states to normalise relations with him. Syrians managed to remind everyone of how shaky his position is and eroded his legitimacy is.”

HTS, which is led by Abu Mohammad al-Jolani, is an offshoot of al-Qaeda affiliate Jabhat al-Nusra, which emerged during Syria’s civil war, but has sought to rebrand itself as a more moderate Sunni Islamist force.

It is listed as a terrorist organisation by the US state department and has controlled one of the armed opposition’s last strongholds in the north-western Syrian region of Idlib. HTS is the most powerful fighting force of the remaining rebel factions.

Neighbouring Turkey, which has backed Syrian rebels since the outset of the Arab state’s civil war, also has troops in northern Syria where it controls large pockets of territory and backs other rebel forces.

Ankara has a relationship with HTS, and although it has less control over the militants and Idlib, it has ultimately acted the protector of the region.

Dareen Khalifa, an adviser at Crisis Group, said Ankara did not encourage the initial HTS offensive.

But she added that the group’s battlefield gains had created an opportunity for Turkey to move its aligned forces into areas of Aleppo province where the Kurdistan Workers’ party (PKK), a Kurdish separatist group that has been fighting the Turkish state for decades, and Iran have a presence.

“It absolutely serves Turkey’s interests. The area has been a massive security headache for them,” she said.

“It’s where the PKK have been having a safe-haven under a kind of Iranian and Russia protection. It’s so close to Turkish-controlled areas, it is completely within their reach.”

Additional reporting by Neri Zilber in Tel Aviv