Two mass graves containing dozens of bodies of civilians and Syrian troops killed by "Islamic State of Iraq and Syria" (ISIS) extremists have been found in the west of al-Raqa province, AFP reported Friday (December 29th).

The bodies were discovered based on information provided by residents near Wawi in the west of the province, state news agency SANA reported. It said they had been executed.

Recovery operations were expected to last several days "due to the huge area of the two mass graves", SANA quoted a source from the Syrian civil defence as saying.

It was not immediately clear when they had been killed, other than while ISIS controlled the province, SANA said.

ISIS, 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.

In December 2014, the bodies of 230 people executed by ISIS were found in a mass grave in the eastern province of Deir Ezzor.

Another 200 civilians, including women and children, were killed by the group in June 2015 in the city of Kobani.