Beirut (AFP) - Islamic State group jihadists have released hundreds of civilians they used as human shields while fleeing a crumbling stronghold in northern Syria, but the fate of others remained unknown Saturday.

On another front, scores of civilians were killed on Saturday in air raids by the Syrian regime and its Russian ally, and in shelling attacks by the rebels in the battleground province of Aleppo, a monitoring group said.

At least 51 civilians including four children were killed in Aleppo city and the surrounding countryside, while another 22 civilians were killed in the neighbouring province of Idlib, the Syrian Observatory for Human Rights said.

The last remaining IS fighters abandoned the city of Manbij near the Turkish border on Friday after a rout the Pentagon said showed the extremists were "on the ropes".

The retreat from the city which IS captured in 2014 was the jihadists' worst defeat yet at the hands of the Syrian Democratic Forces (SDF), an Arab-Kurdish alliance backed by US air power.

Fleeing fighters took around 2,000 civilians, including women and children, Friday to ward off air strikes as they headed towards the IS-held frontier town of Jarabulus, according to the SDF.

At least some captives were later released or escaped, the alliance said Saturday, but the whereabouts of the rest was unknown.

"There are no more IS fighters" left in Manbij, an SDF member said.

Kurdish television showed footage of jubilant civilians in Manbij, including smiling mothers who had shed their veils and women embracing Kurdish fighters.

One woman burned a black robe that the jihadists had forced residents to wear, while men who had lived for weeks under a shaving ban cut their beards.

- Booby-trapped houses -

"The battle was very hard," a Kurdish source told AFP, adding the jihadists had laid mines in the city.

"One SDF fighter entered a house on Friday and saw a shoe placed on a Koran. When he removed it there was an explosion and he was killed," the source said.

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One resident told AFP there was not a single house inside his neighbourhood that had not been booby-trapped.

"We ask the people in charge... to do something" to remove the mines, Jamal Abul Ababiyya said, adding that mines were wounding people every day.

AFP footage showed the city's streets strewn with rubble and a wall still painted with the jihadists' black and white flag.

A female SDF fighter, a traditional keffiyeh scarf loosely tied around her hair, beamed as she spoke of the city's "liberation".

"We're inside Manbij after its liberation from IS... and taking civilians to secure places," Nayruz Serekaniye said.

The Britain-based Observatory also reported that several hundred of the civilians taken from Manbij were no longer being held by IS.

"Among the civilians taken by IS there were people used as human shields but also many who chose voluntarily to leave the town due to fear of reprisals" by the SDF, Observatory head Rami Abdel Rahman said.

The SDF began an assault in May on Manbij, on a key jihadist supply route between the Turkish border and IS's de facto Syrian capital Raqa.

The jihadists, who have suffered a string of losses in Syria and Iraq, have often staged mass abductions when they come under pressure to relinquish territory they hold.

IS has also booby-trapped cars and carried out suicide bombings to slow advances by its opponents.

- Hundreds killed -

SDF forces captured Manbij on August 6 but continued to battle pockets of jihadist resistance there.

According to the Observatory, 437 civilians, including more than 100 children, were killed in the battle for Manbij and surrounding territory.

Around 300 SDF fighters died, along with more than 1,000 jihadists, it said.

Pentagon deputy press secretary Gordon Trowbridge said on Friday that IS "is clearly on the ropes".

"It has lost the centre of Manbij, it has lost control of Manbij," he said.

Since the battle for Manbij began, US-led strikes have destroyed more than 50 IS heavy weapons and more than 600 fortified fighting positions, Trowbridge said.

But the job of clearing the city will be difficult after the jihadists left behind hundreds of mines and booby traps, he added.

Syria's conflict erupted in March 2011 and has since killed more than 290,000 people and drawn in world powers on all sides of the war.

Rebels and government forces clashed Saturday in Syria's second city Aleppo, a fortnight on from the launch of a major rebel offensive on July 31.

In Aleppo city and the neighbouring countryside, air raids by the Syrian regime and its Russian ally, as well as shelling by the rebels left at least 51 civilians dead on Saturday, according to the Observatory.

The latest toll brought to over 230 the number of civilians killed in the embattled city since the rebel offensive began.

Strikes by the Russian and Syrian air force continued despite a pledge by Moscow to observe a three-hour daily ceasefire to allow humanitarian aid deliveries.