At least 13 people, including nine children, were killed in air strikes on the rebel-held east of Aleppo on Friday, the Syrian Observatory for Human Rights monitor said.

The Britain-based group said it was unclear if the raids on the Marjeh district were carried out by aircraft belonging to the Syrian government or its ally Russia.

The latest civilian deaths come as rebels press an assault intended to break a government siege of opposition-held Aleppo that began on July 17 and has raised fears of a humanitarian crisis.

In an audio message released on Friday, Abu Mohamed al-Jolani, the head of Al-Qaeda's former affiliate in Syria, pledged that the operation would soon succeed.

The rebel assault started on Sunday and is targeting the Ramussa district that contains the main supply route to the government-held west of the city.

After some initial rebel advances, government forces backed by Russian air strikes have pushed back opposition fighters who include jihadists from Jolani's faction, now known as the Fateh al-Sham Front.

On Friday, rebels launched a new offensive targeting a military academy east of Ramussa. Despite fierce fighting, they made little progress against regime forces, said the Observatory's director Rami Abdul Rahman.

Once Syria's economic powerhouse, Aleppo has been ravaged by the war that began in March 2011 with anti-government protests.

According to the Observatory, at least 115 civilians, including 35 children, have been killed in Aleppo 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, the monitor said.

It reported five more deaths in rebel fire on the Kurdish-majority Sheikh Maqsud district of the city.

In an audio message, Jolani said God had granted fighters "a glorious victory in the battle to break the siege of Aleppo".

"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," he said.

Elsewhere in Aleppo province, the monitor said an alliance of Kurdish and Arab fighters was advancing inside the Islamic State 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.