A mass grave filled with the bodies of 3,500 people has been found in the former ISIS stronghold of Raqqa.

First responders found two foot deep graves just outside the Syrian city in the al-Fukheikha agricultural suburb last month, more than a year after US-backed forces captured the city from ISIS.

Experts believe the site, the largest mass grave found in the region, could help identify several thousand people who are believed to have been murdered by ISIS, but whose fate remains unknown.

The Raqqa civil defence gather bodies after excavating the site of a mass grave near the northern Syrian city

Shallow graves measuring just two foot were used to dump the bodies in, as a dispatch team now work to identify the victims

'We've heard accounts from residents of al-Fukheikha that they would see people gathering with someone in an orange suit,' said one expert.

US journalist James Foley was wearing such a suit when he was executed by ISIS fighters, and analysts believe he may have been killed around Raqqa. Kayla Mueller, another American hostage, is also thought to have died in the region. Neither body has been recovered.

Sara Kayyali of Human Rights Watch said: 'These mass graves hold the answers to the fate of people who had been executed by IS fighters, who died in coalition air strikes, or who have been missing.'

The latest discovery is the largest mass grave found in the region and is another example of how ISIS's reign of terror will be harvested for years to come, diggers and activists have said.

The latest discovery is the largest mass grave found in the region and is another example of how ISIS's reign of terror will be harvested for years to come, diggers and activists have said

Several dozen mounts of dirt lined one side of the plot, where more than 120 bodies have already been dug up by the Rapid Response Division of Raqqa's civil defence service

Several dozen mounds of dirt lined one side of the plot, where more than 120 bodies have already been dug up by the Rapid Response Division of Raqqa's civil defence service.

'These are individual graves, but behind us, by the trees, are the mass graves of those executed by Daesh (IS),' said Asaad Mohammad, the 56-year-old forensic assistant at the site.

'There are some 2,500-3,000 bodies estimated there, plus between 900 and 1,100 bodies in the individual graves, so at least 3,500 total.'

Eight other mass graves have been identified around northern Syria, from which 900 bodies have been exhumed.

Some of the bodies were so badly decomposed that the bags had to be weighted down with stones to stop them flying away in the wind.

Since the Raqqa division began digging up graves in January 2018, it has recovered more than 3,800 bodies. Just 560 have been formally identified and handed over to families for a proper burial, the force's supervisor Turki al-Ali said.

Some of the bodies were so badly decomposed that the bags had to be weighted down with stones to stop them flying away in the wind