BEIRUT: The militant Islamic State (IS) group fighters seized around 2,000 civilians to use as “human shields” Friday as they fled their stronghold of Manbij in northern Syria, US-backed forces and a monitor said.

The abductions came as Russian and Syrian jets pounded militant positions in second city Aleppo, an AFP correspondent and the Syrian Observatory for Human Right monitor said.

The Arab-Kurdish alliance known as the Syrian Democratic Forces (SDF) expelled most of the IS fighters from Manbij last week, but dozens continued to put up a tough resistance.

On Friday they withdrew from a district in northern Manbij heading for the IS-held town of Jarabalus along the border with Turkey, taking the captives with them.

“While withdrawing from a district of Manbij, Daesh (IS) jihadists abducted around 2,000 civilians from Al-Sirb neighbourhood,” said Sherfan Darwish, spokesman for the Manbij Military Council, a key component of the SDF.

“They used these civilians as human shields as they withdrew to Jarabulus, thus preventing us from targeting them,” he said, adding that women and children were among those taken.

The Britain-based Observatory, which relies on sources on the ground to cover the conflict, also reported that IS had abducted around 2,000 civilians as they fled Manbij.

It said IS confiscated residents' cars, forced civilians into them and then headed for Jarabulus.

The militants, who have suffered a string of losses in Syria and Iraq, have often staged mass kidnappings in the two countries when they come under pressure to relinquish territory they hold.

In January, IS abducted more than 400 civilians, including women and children, as it overran parts of Deir Ezzor province in eastern Syria. It later released around 270 of them.

IS has also used civilians as human shields, booby-trapped cars and carried out suicide bombings to slow advances by their opponents and avoid coming under attack.

Thousands of civilians were held captive by the group in Fallujah which Iraqi forces recaptured in June after a four-week offensive.

On Friday, the SITE intelligence Group said IS had killed five men in Iraq for smuggling people out of territory it controls.

It was not immediately clear how many militants fled Manbij which the SDF captured on August 6.

But Darwish said that SDF managed to rescue 2,500 civilians who had been held captive by IS fighters there.

The US-backed forces combed Al-Sirb on Friday for any remaining militants, he added.

With air support from the US-led coalition, the SDF began its assault on Manbij on May 31, surging into the town itself three weeks later.

But their offensive was slowed by a massive militant fightback, before a major push last week saw the SDF seize 90 per cent of the town.

Tens of thousands of people lived in Manbij before the assault started in May. The United Nations has said that more than 78,000 people have been displaced since then.

Manbij was a key transit point along IS's supply route from the Turkish border to Raqa, the de facto capital of its self-styled Islamic “caliphate”.

The Britain-based Observatory says the battle for Manbij claimed the lives of at least 437 civilians - including 105 children - and killed 299 SDF fighters and 1,019 militants.

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.

On Friday, Turkey vowed to cooperate with Syrian ally Iran to try to resolve the conflict in which Ankara backs rebels battling the government of President Bashar al-Assad.