Syria's regime and rebels were locked in fierce fighting Sunday on Aleppo's western edges, where 38 civilians have been killed in a two-day opposition offensive to break the government siege.
Rebels and allied militants launched a major offensive on Friday to break through government lines and reach the 250,000 people living in the city's east.
Since then, they have unleashed a salvo of rockets, artillery shells, and car bombs around the western government-controlled districts.
Syria's second city, Aleppo has been devastated by some of the heaviest fighting of the country's five-year civil war, which has killed more than 300,000 people.
Much of the once-bustling economic hub has been reduced to rubble by air and artillery bombardment, including barrel bombs -- crude unguided explosive devices that cause indiscriminate damage.
"Rebel fighters have launched hundreds of rockets and shells onto the western districts from positions inside the city and on its western edges," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.
Two days of such heavy rebel bombardment have killed 38 civilians, including 14 children, and wounded another 250.
Fighting has also killed 30 regime and allied fighters, as well as 50 Syrian rebels, according to the Observatory.
The monitor did not have an immediate death toll for foreign anti-regime fighters, many of whom have joined the militants' factions.
About 1,500 rebels have massed on a 15-kilometre front along the western edges of Aleppo since Friday, scoring quick gains in the Dahiyet al-Assad district but struggling to push east since then.
"The advance will be from Dahiyet al-Assad towards Hamdaniyeh," said Yasser al-Youssef of the Noureddin al-Zinki rebel faction.
Hamdaniyeh is a regime-held district directly adjacent to opposition-controlled eastern neighbourhoods.
Fighting lasted all night and into Sunday, with air strikes and artillery fire along the western battlefronts heard even in the eastern districts, an AFP correspondent there said.
Plumes of smoke could be seen snaking up from the city's skyline.
A pro-regime military source told AFP that the rebel assault was "massive and coordinated" but insisted it was unable to break into any neighbourhoods besides Dahiyet al-Assad.
"They're using Grad missiles and car bombs and are supported by foreign fighters in their ranks," he said.
Those waging the assault include Aleppo rebels and reinforcements from Idlib province to the west, among them the militant Fateh al-Sham Front, which changed its name from Al-Nusra Front after breaking ties with Al-Qaeda.
Aleppo's front line runs through the heart of the city, dividing rebels in the east from government troops in the west.
In late September, government troops launched their own assault to recapture all of the eastern rebel-controlled territory.
It was backed by fierce air strikes from Russia, which launched its own air war in 2015 to back President Bashar al-Assad's forces.
That onslaught spurred massive international criticism of both Moscow and Damascus.
Last week, Russia implemented a three-day "humanitarian pause" intended to allow civilians and surrendering rebels to leave Aleppo's east, but few did so.
Moscow says it will continue a halt on air strikes over Aleppo, in place since October 18.
The Russian military said Friday it had asked President Vladimir Putin for authorization to resume the raids.
But Kremlin spokesman Dmitry Peskov said Putin "considers it inappropriate at the current moment", adding that the president thought it necessary to "continue the humanitarian pause" in Aleppo.