Russian and Syrian warplanes launched airstrikes on rebel-held areas in Syria's northwest after insurgents, led by Hayat Tahrir al-Sham, advanced into Aleppo. This marks a significant escalation in the Syrian civil war, challenging Assad's control.

AMMAN (Reuters) - Russian and Syrian warplanes targeted insurgents in an Aleppo city suburb on Saturday, two Syrian military sources said, after the rebel fighters penetrated the heart of the city in Syria's northwest in a surprise attack the previous day.

The attack by the Islamist militant group Hayat Tahrir al-Sham marks the most significant challenge in years to President Bashar al-Assad, jolting the frontlines of the Syrian civil war that have largely been frozen since 2020.

The Syrian Civil Defense, a rescue service operating in opposition-held parts of Syria, said in a post on X that Syrian government and Russian aircraft carried out airstrikes on residential neighbourhoods, a gas station and a school in rebel-held Idlib, killing four civilians and wounding six others.

Russia, which deployed its air force to Syria in 2015 to support Assad in the war, has promised Damascus extra military aid to thwart the rebels, the two military sources said, adding this would start arriving in the next 72 hours.

The insurgent force began its surprise offensive earlier this week, sweeping through government-held towns and reaching Aleppo nearly a decade after government forces backed by Russia and Iran drove rebels from the city.

Syrian authorities closed Aleppo airport as well as all roads leading into the city on Saturday, the two military sources and a third army source said.

The Syrian army has been told to follow "safe withdrawal" orders from the main areas of the city that the rebels have entered, the three military sources said.

On Friday, Syrian state television denied rebels had reached the city and said Russia was providing Syria's military with air support.

The Syrian military said on Friday it was fighting back against the attack and had inflicted heavy losses on the insurgents in the countryside of Aleppo and Idlib.

Also speaking on Friday, Kremlin spokesman Dmitry Peskov said Moscow regarded the rebel attack as a violation of Syria's sovereignty.

"We are in favour of the Syrian authorities bringing order to the area and restoring constitutional order as soon as possible," he said.

The rebels began their incursion on Wednesday under the umbrella of an operations room, including groups that are backed by Turkey. On Friday, the operations room said its forces were sweeping through various neighbourhoods of Aleppo.

Assad recovered full control of Aleppo city from rebel forces in 2016, aided by Russian air power and Iranian-backed Shi'ite militias, with the insurgents agreeing to withdraw after months of bombardment and siege.

Insurgents maintained a foothold near Aleppo in Idlib province and in areas north of the city at the Turkish border.

SPILLOVER EFFECT OF GAZA WAR

Mustafa Abdul Jaber, a commander in the Jaish al-Izza rebel brigade, said their speedy advance this week had been helped by a lack of Iran-backed manpower in the broader Aleppo province. Iran's allies in the region have suffered a series of blows at the hands of Israel as the Gaza war has expanded through the Middle East.

The opposition fighters have said the campaign was in response to stepped-up strikes in recent weeks against civilians by the Russian and Syrian air force on areas in rebel-held Idlib, and to preempt any attacks by the Syrian army.

Opposition sources in touch with Turkish intelligence said Turkey, which supports the rebels, had given a green light to the offensive.

Turkish officials were not immediately available to comment on Saturday.

Turkey's foreign ministry said on Friday that clashes between rebels and government forces in the northwest had resulted in an undesirable escalation of tensions.

In a statement on Friday, spokesperson Oncu Keceli said that avoiding greater instability in the region was Turkey's priority, adding that Ankara had warned that recent attacks on Idlib undermined the spirit and implementation of de-escalation agreements.

The attack by insurgents is the biggest since March 2020, when Russia and Turkey agreed to a deal to de-escalate the conflict.

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