An opposition assault to break the siege of Syria's Aleppo slowed Monday (October 31st) amid fierce resistance, as the UN said it was "appalled" by opposition fire on civilians, AFP reported.

Opposition factions allied with extremist groups – including al-Nusra Front (ANF), now known as Fatah al-Sham Front – on Friday launched a major assault , backed by car bombs and rocket fire, to break through regime lines and reach the 250,000 people besieged in the city's east.

While they scored an initial advance, the offensive has since slowed, according to the Syrian Observatory for Human Rights.

"Since Sunday, the regime has been taking the initiative and the clashes are less intense," Observatory head Rami Abdel Rahman said Monday.

In a new toll on Monday, the Observatory said a total of 61 regime fighters and allied militiamen were killed in the assault, as well as 72 Syrian opposition fighters.

Heavy opposition rocket fire since Friday has killed 48 civilians, including 17 children, the Observatory said.

UN peace envoy Staffan de Mistura said the high civilian toll raised deep concern.

In a statement on Sunday, his office said he was "appalled and shocked by the high number of rockets" fired by opposition factions.

"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.