The organisation is the leading militia in the north-eastern part of rebel-held Syria, and has taken control of parts of the civilian administration in the city of Raqqa.

It posted a new notice there this week saying it was open to receiving complaints from members of the public.

“Any one who might have a complaint against any element of the Islamic state, whether the Emir or an ordinary soldier, can come and submit their complaint in any headquarters building of the Islamic state,” the notice said. “The complaint should be in writing, provide details and give evidence.

“We promise that we will ensure accountability for anyone committing violations, and they will be sent to the Sharia court of Iraq and al-Sham.”