Russian warplanes pounded the southern edges of Syria’s Aleppo city overnight, slowing a “last chance” rebel offensive against President Bashar al-Assad’s forces, a monitor said Tuesday.

Jihadists and rebel groups launched a major assault Sunday on the southern edges of the divided city in a bid to break a government siege of eastern opposition-held neighbourhoods.

But government fighters backed by Russia’s air force have put up a fierce defence of the southwestern outskirts.

“The Russian raids didn’t stop all night on the front lines” there, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, a British-based monitoring group.

“This has slowed the offensive and allowed regime troops to retake five of the eight positions that rebels had taken since Sunday,” he added.

Abdel Rahman said opposition fighters from the Fateh al-Sham Front, formerly Al-Qaeda affiliate Al-Nusra Front, and allied Islamists were struggling to hold positions they had seized.

The primary goal of the rebel assault is to seize the Ramussa district on the city’s southern outskirts, used as the main access route for regime forces and civilians living in government-controlled parts of Aleppo.

Capturing Ramussa would simultaneously cut off government forces and give rebels a new access route to their besieged neighbourhoods in east Aleppo.

The Observatory said 50 rebels and allied jihadists had been killed since the operation began on Sunday, as well as dozens of regime troops.

At least 30 civilians have been killed since Sunday in opposition bombardment of government-held southwestern districts of Aleppo, the monitor said.

The city was once Syria’s economic powerhouse but has been ravaged by fighting, particularly in recent months as rebels and the regime each try to assert control.

According to a Syrian military source, about 5,000 pro-regime fighters, including Iranian forces and the powerful Lebanese Shiite movement Hezbollah, are taking part in the battle for the city, including fighting north of Aleppo.

Facing off against them are thousands of fighters from the Fateh al-Sham Front and allied Islamists including the powerful Ahrar al-Sham faction.

The Observatory called it the largest assault by rebel forces in Aleppo since 2012, when violence first broke out there and opposition fighters seized half the city.

“This battle is the last chance for rebels. If they lose, it will be difficult for them to launch a new assault to break the siege,” Abdel Rahman said.

More than 280,000 people have been killed and millions have been forced to flee their homes since Syria’s conflict erupted in March 2011.