Exhumation work is under way at a mass grave in Syria's al-Raqa province, discovered in mid-April, that contains the remains of victims executed by the "Islamic State of Iraq and Syria" (ISIS), a civil defence volunteer said.

Initial Response Teams and al-Raqa's civil defence are currently establishing a cordon around the Tal Zaidan area in eastern rural al-Raqa province where the grave was found, said al-Raqa civil defence volunteer Hamad al-Matar.

Work is under way to exhume the bodies from the grave site, he told Diyaruna, albeit at a slow pace, due to difficulties the teams have encountered.

The mass grave, located between the al-Raqa towns of al-Samra and al-Hamrat, contains the remains of ISIS victims who were deposited there after the group carried out death sentences it had issued against them, al-Matar said.

The mass grave is one of 25 ISIS left behind in the area, he said.

"It is important to locate and exhume these mass graves and verify the identities of the victims, in order to resolve the fate of thousands of people who vanished years ago after ISIS overran the area," he said.

Al-Matar said the grave site, discovered in April, is located near the agricultural areas in eastern al-Raqa.

"It is clear that the ISIS executioners threw the bodies into the grave in haste, judging by how they were found," he said -- piled in on top of each other.

This grave site was most likely active towards the end of ISIS's occupation, he said -- just before it was expelled from the area by the international coalition and the Syrian Democratic Forces (SDF).

Al-Matar estimated there are more than 200 bodies interred at the site, and said it will take at least two months or more to exhume all of them, due to the difficulty of the work involved.

Team members are working at the site each day, exhuming an average of two to three bodies a day -- most of them men -- he said.