In an open field on the outskirts of the Syrian city of Raqqa, workers in black uniforms and red caps work under a scorching sun to dig up bodies from a large mass grave discovered last month.

They have so far unearthed 313 bodies from the grave since it was discovered, an official said. All of the dead are men, women and children believed to have been killed or died during the Islamic State in Iraq and Syria (ISIS) rule over the northern city, once the de facto capital of the extremist group's so-called Islamic caliphate and the site of atrocities committed by the group against residents who opposed its extremist ideology.

The group at the time commanded large swaths of territory in Syria and Iraq that it administered based on its own radical version of Islamic rule.

U.S.-backed Syrian forces retook Raqqa from ISIS in 2017 after a lengthy campaign that left the city in ruins. Since then, an organization known as the Civil Council of Raqqa has been working to uncover mass graves in and around the city, amid concerns about the preservation of bodies and evidence for possible war crimes trials.

International human rights groups say they are concerned that local groups are not getting the support they need in terms of forensic expertise and human resources.

Yasser al-Khamees, who leads a team of first responders, said workers have unearthed 4,760 bodies from a series of mass graves starting from January 2018.

The mass grave discovered in mid-June in al-Fukheikha agricultural fields south of the city is the latest grave located in and around Raqqa to date.

An Associated Press video showed workers on a recent day wearing surgical masks using shovels to dig up bodies from the field. Forensic workers then put the remains in white body bags, marking them with the date and location and other details.

Asaad Mohammed, a forensic worker, said workers were exhuming an average of 10 to 12 bodies each day.

"We inspect the body, identify the sex, age, time of death, cause of death. We take samples from each body and give it a number, we document it on official papers and then save the information on a computer database along with samples taken from this area," he said.

The bodies are then reburied in another plot of land outside the city.