A rebel fighter fires mortar shells towards western government-controlled districts on October 30, 2016 at an entrance to Aleppo on the third day of a rebel offensive to break a three-month siege of the opposition-held east of Syria's second city
Beirut (AFP) - A rebel assault to break the siege of Syria's Aleppo slowed Monday amid fierce resistance from regime forces, as the UN said it was "appalled" by opposition fire on civilians.
Rebels launched a major assault Friday, backed by car bombs and salvos of rockets, to break through government lines and reach the 250,000 people besieged in the city's east.
Aleppo has been hit by some of the worst violence in Syria's five-year conflict, turning the once bustling economic hub into a divided and bombed-out symbol of the war.
Since Friday, opposition factions allied with jihadists have amassed on Aleppo's western outskirts in a bid to end the regime's three-month encirclement of the city's eastern districts.
While they scored an initial advance, the offensive has since slowed, according to the Syrian Observatory for Human Rights, a Britain-based monitor.
"Since Sunday, the regime has been taking the initiative and the clashes are less intense," Observatory head Rami Abdel Rahman said on Monday.
"The only thing that has been accomplished is partial control over Dahiyet al-Assad," a neighbourhood on Aleppo's western outskirts that rebels entered on Friday, he said.
Regime and Russian air strikes were hitting the battlefronts on the city's edges, but with less intensity than in previous days.
"The momentum of the rebel offensive slowed after failing to take control of the '3000' apartment block and the military complex," a pro-regime military source said, referring to two built-up areas southwest of Aleppo.
- Civilian toll rises -
The Observatory said 61 regime fighters and allied militiamen were killed in the assault since Friday, as well as 72 Syrian rebels.
And in a new toll Monday, the monitor said nearly four days of heavy rebel rocket fire have killed 51 civilians including 18 children.
Syria's state news agency SANA said three civilians were killed in rebel fire Monday.
The army gave a toll of 84 people killed in three days, "mostly women and children," repeating allegations that rebels had fired shells containing chlorine gas on western Aleppo.
Rights groups Amnesty International said rebels had "displayed a shocking disregard for civilian lives".
"The goal of breaking the siege on eastern Aleppo does not give armed opposition groups a license to flout the rules of international humanitarian law," said Amnesty's Samah Hadid.
UN peace envoy Staffan de Mistura said Sunday he was "appalled and shocked by the high number of rockets" fired by rebels.
"Those who argue that this is meant to relieve the siege of eastern Aleppo should be reminded that nothing justifies the use of disproportionate and indiscriminate weapons, including heavy ones, on civilian areas and it could amount to war crimes," he said.
"Civilians of both sides of Aleppo have suffered enough due to futile but lethal attempts of subduing the city," he added.
Aleppo's front line runs through the heart of the city, dividing rebels in the east from government forces in the west.
Rebel groups have pledged to push east from Dahiyet al-Assad to Hamdaniyeh, a regime-controlled neighbourhood adjacent to the besieged eastern districts.
Sarab Abu Abdo, a rebel commander in the Army of Conquest alliance, said fighting was "ongoing with light weapons" Monday.
- Ambush in south -
"We seized three blocks of the 3000 apartment complex, but the regime still controls most of it," Abu Abdo said.
He said regime forces had tried twice to overrun Minyan, a village west of Aleppo captured by rebels Saturday, but failed.
An AFP correspondent saw about a dozen civilians, including women and children, fleeing Dahiyet al-Assad Sunday.
They carried belongings stuffed into plastic bags over their heads or dragged them along the dusty road.
Syria's conflict broke out in March 2011 with protests calling for the ouster of President Bashar al-Assad.
It has evolved into a complex, multi-front war pitting regime forces, rebels, Kurds and jihadists -- including the Islamic State group -- against each other.
Turkey said Monday the drive to oust IS from Syria's northern city of Raqa should begin after the end of the offensive on Mosul, the last IS-held city in Iraq.
"It would be right, militarily and strategically, to conduct this Raqa operation after the Mosul operation and Turkey's Euphrates Shield operation have ended," Deputy Prime Minister Numan Kurtulmus said.
The UN Security Council Monday extended the mandate of a panel investigating chemical attacks in Syria for two weeks to negotiate a one-year renewal of the probe.
The United Nations-Organization for the Prohibition of Chemical Weapons (OPCW) panel found government forces carried out three chlorine gas attacks in 2014 and 2015.