Syrian workers have exhumed more than 500 bodies from one of the largest mass graves near the northern city of Raqqa, once the capital of the Islamic State in Iraq and Syria's self-styled caliphate, and are still uncovering remains, a local official said Tuesday.

The exhumation of mass graves in and around Raqqa is being undertaken by local groups and first responders amid concerns about the preservation of bodies and evidence for possible war crimes trials.

A U.S.-backed air and ground campaign drove ISIS from Raqqa more than a year ago, but rescuers and recovery teams continue to locate mass graves in and around the city.

The Panorama mass grave, named after the neighbourhood where it was found, is one of the largest of nine mass graves discovered so far, and is believed to contain around 1,500 bodies. Hammoud al-Shawakh, a local official involved in the work, said 516 bodies believed to be of ISIS fighters and civilians have been exhumed.

A team of Raqqa-based first responders and a forensic doctor carefully shovel dirt to search for the bodies, which are believed to have been buried there in the last days of the four-month campaign to liberate Raqqa.

Abdul Raouf al-Ahmad, the deputy forensic doctor, said local teams start their work at 8 a.m. and work for more than seven hours straight each day.

"After we extract the bodies from this grave … we document whether it belongs to a fighter, child, baby, an adolescent or woman or an ordinary person," he said. "We document clothing, ornaments, height, type of injury, cause of death and how it was covered, what the person was wearing, with what it was wrapped and its position in the grave."

An image made from video shows Syrian workers walking through the mass grave in Raqqa. (Associated Press)

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.

"We're in a race against time. These bodies are decomposing at an exponential rate," said Sara Kayyali, of Human Rights Watch.

"If these bodies are not preserved in the correct way, in the way that's been established, then it does mean that much of this evidence might be lost when we're seeking accountability for crimes committed either in the context of the battle or before it," she added.