A mass grave containing the bodies of dozens of people thought to have been Yazidis enslaved by the Islamic State (IS) group has been found in territory recently seized by the US-backed Syrian Democratic Forces.

Key points: Many of the bodies discovered in the mass grave were women, and most were beheaded

Many of the bodies discovered in the mass grave were women, and most were beheaded Thousands of members of the Yazidi minority sect from Iraq were forced into sexual slavery by jihadists

Thousands of members of the Yazidi minority sect from Iraq were forced into sexual slavery by jihadists The grave was discovered by US-backed Syrian forces evacuating civilians before an attack IS territory

Many of the bodies found in the Baghouz area were those of women.

"They were slaughtered," SDF commander Adnan Afrin said.

Most had been decapitated, he added.

The SDF is trying to confirm if the bodies belonged to members of the Yazidi sect.

Thousands of members of the minority sect from Iraq were forced into sexual slavery by the jihadists when they surged across the border in 2014 and seized swathes of territory.

IS members could be seen walking around in Baghouz on February 18. ( Reuters: Rodi Said )

More than 3,000 other Yazidis were killed in an onslaught the United Nations later described as genocidal, which prompted the first US air strikes against IS.

Thousands more fled on foot, and many of them remain displaced more than four years later.

Thousands flee as fighting closes in

The SDF, the main Kurdish led-partner of the US-led coalition against IS in Syria, has been trying to evacuate thousands of civilians in Baghouz before storming it, or forcing the surrender of the remaining jihadists, who the SDF has said were mostly foreigners.

Civilians were evacuated in trucks as the SDF forces closed in on IS fighters. ( Supplied: Free Burma Rangers )

"Our forces have managed to save a large number of civilians, along with our comrades who have been held hostage by IS," an SDF spokesperson said.

"Rescued civilians were provided with safe passage and transferred to elsewhere."

The SDF said earlier on Thursday that it had freed 24 of its fighters held by militants in Baghouz, but gave no details about how they were recovered.

According to an SDF statement, the fighters were in "good health" but were malnourished "because of intended negligence by the terrorist organization".

Thousands of people of many nationalities — both IS supporters and victims — have streamed out of the area in recent weeks, surpassing initial estimates and delaying an end to the battle.

Residents say hundreds of Arab civilians have been killed in US-led coalition air raids since the campaign began several months ago, with many of their homes in villages and towns east of the Euphrates River levelled to the ground.

The United Nations on Thursday expressed concern about the plight of thousands of people who residents say are tribal Arabs in a camp in al Hol, in northeastern Hasaka province.

Many of those fleeing, nearly all of them women and children, were being taken into custody by the SDF.

Evacuees were screened by SDF forces after being trucked out of the conflict zone. ( AP: Felipe Dana )

Those fleeing described a desperate situation for civilians, with widespread reports of civilians killed and injured, destruction of civilian infrastructure, and shortages of food, a UN statement issued in Damascus said.

The United Nations said more than 78 deaths had been reported, two-thirds of them children under one year old who lost their lives either en route or shortly after arriving at the SDF-run camp.

ABC/Reuters