Jihadists and their rebel allies pressed an offensive Friday with a massive attack aimed at seizing a military academy south of Aleppo and breaking a three-week-old siege of insurgent neighbourhoods.
Their assault focusing on the artillery and armaments schools of the academy has sparked contradictory reports about the situation on the ground.
"The rebels have seized parts of these schools, but the army has begun a counter-offensive backed by air cover to chase them out," said Syrian Observatory for Human Rights chief Rami Abdel Rahman.
"If the rebels can take control of these schools, they will cut the supply route into regime-held districts of western Aleppo and they must seize the adjoining area of Ramussa to lift the siege on the rebel districts," he said.
"This is a battle of life and death for the regime and for Russia," its key ally, Abdel Rahman added.
State television in Syria said government forces had repulsed the assault, killing "hundreds" of attackers.
Once Syria's economic powerhouse, Aleppo has been ravaged by the war that began in March 2011 with anti-government protests.
The government siege of opposition-held districts began on July 17 and has raised fears of a humanitarian crisis.
The Observatory said at least 13 people, including nine children, were killed in air strikes on the rebel-held east of the city on Friday.
The Britain-based group said it was unclear if the raids on the Marjeh district were carried out by Syrian or Russian aircraft.
Moscow, meanwhile, hit back at US criticism over Syria, accusing Washington of lacking trust in Russia and not behaving as a true partner.
Russia and the United States support opposing sides in Syria's civil war, which has left 280,000 people dead and forced half the population to flee their homes.
'Wholly logical' policy
But the two world powers have agreed "concrete steps" to revive a tattered ceasefire and tackle jihadist groups in Syria, although details have not been made public.
On Thursday, US President Barack Obama said Russia risked casting itself as an "irresponsible actor" on the world stage particularly due to its actions in Aleppo in recent weeks.
"The United States sometimes don't behave as partners when dealing with us on Syrian issues and are not always ready to negotiate on equal grounds," Deputy Foreign Minister Sergei Ryabkov told TASS news agency.
He said he thought Moscow's policy on Syria was "wholly logical" and set goals that are mostly "shared by most members of the international community".
The battle for northern Syria.
In an audio message released Friday, Abu Mohamed al-Jolani, the head of Al-Qaeda's former affiliate in Syria, pledged that the rebel offensive which started on Sunday would soon succeed.
God had granted fighters "a glorious victory in the battle to break the siege of Aleppo", he said.
"This battle, the outcome of which will go beyond simply opening the road for the besieged, will change the balance of the conflict... setting the scene for a new stage of the battle."
After some initial rebel advances, government forces backed by Iranian and Hezbollah fighters as well as Russian air strikes have pushed back opposition fighters who include jihadists from Jolani's faction, now known as the Fateh al-Sham Front.
Ravaged by war
According to the Observatory, at least 115 civilians, including 35 children, have been killed in the city since the rebel assault began on Sunday.
The deaths include 65 people, among them 22 children, killed in rebel fire on government neighbourhoods, the Observatory said.
Another 42 people, including 11 children, have been killed in strikes on eastern Aleppo, it said.
It reported five more deaths in rebel fire on the Kurdish-majority Sheikh Maqsud district of the city.
Elsewhere in Aleppo province, the monitor said an alliance of Kurdish and Arab fighters was advancing inside the Islamic State group bastion of Manbij.
Abdul Rahman said the Syrian Democratic Forces (SDF) now held 70 percent of the town.
"The Islamic State has entered its final phase in the town of Manbij," he told AFP, adding however that IS was using a number of civilians there as human shields.
"The progress the SDF has made in the last week is more than it was able to achieve in previous weeks put together," Abdul Rahman said.
The SDF began its assault on Manbij in late May and entered the town proper on June 23, with support from the US-led coalition against IS.
Manbij sits on the route between the Turkish border and the eastern city of Raqa, the jihadists' de facto capital in Syria.