Dozens of bodies, including those of jihadists and civilians, have reportedly been found in a mass grave in a former stronghold of the so-called Islamic State group in Raqa in Syria.

An official with Raqa Civil Council, which now runs the city, said nearly 50 bodies had been recovered from the mass grave, which it is believed could contain up to 200 bodies.

It was located under a football pitch, close to a hospital where IS militants had dug in before they were ousted by American-backed Syrian Democratic Forces in October last year.

"It was apparently the only place available for burials, which were done in haste. The jihadists were holed up in the hospital," the official said, adding that some bodies were marked with the nom de guerre of the jihadist while civilians just had first names.

In recent months, both Syria and Iraq have discovered mass graves in areas previously occupied by the jihadists.

Syrian troops uncovered a mass grave containing the remains of more than 30 people killed by IS in Raqa province in February.

It followed two other similar finds by the Syrian army.

The Islamic State group, which proclaimed a "caliphate" over swathes of Syria and Iraq in 2014, has now lost almost all the land it once controlled.

It has been held responsible for multiple atrocities during its reign of terror, including mass executions and decapitations.