A hardline Islamist rebel group in Syria has released a video showing its fighters shooting dead a group of alleged Islamic State (IS) militants, BBC News reports.

Reversing the imagery of beheadings by the jihadist group, Jaysh al-Islam members wearing orange outfits are seen killing 18 men dressed in black.

The video says they are being shot in revenge for the recent beheading of three Jaysh al-Islam fighters. The Saudi-backed group also accuses IS of being allied to Syria's government.

Eighteen months of fierce fighting between IS and rebel groups, who have denounced the jihadists' brutal tactics and mistreatment of civilian opposition supporters, have left thousands dead.

Led by the former imprisoned Salafist activist Zahran Alloush, Jaysh al-Islam (Army of Islam) has been battling IS in the Ghouta agricultural belt around Damascus.