Islamic State (ISIS) jihadists beheaded 12 men from rival Syrian rebel movements accused of fighting against them, in a video released on Thursday, according to AFP.

It is the latest in a long series of mass beheadings by ISIS, and comes two days after the group released a video showing it killing 16 people in neighboring Iraq, drowning some of them in a cage.

Three of those killed in the new video were from Jaysh al-Islam, one of the main rebel groups in the Damascus area, and a fourth from Al-Nusra Front, Al-Qaeda's Syrian affiliate and ISIS’ main jihadist rival in the country.

While the men, whom ISIS said were captured during a battle, were apparently from Islamist Syrian rebel groups, their heads and beards were shaved, likely as an insult by ISIS, noted AFP.

Powerful jihadist rebel groups in Syria, including the Islamic Front and Al-Nusra Front, became locked in fierce fighting with ISIS after allegations that ISIS brutally abused civilians and other opposition fighters battling to topple President Bashar AlAssad.

Tensions were further exacerbated when Al-Nusra and other rebel factions accused ISIS of killing Islamist Front commander Abu Khaled al-Suri in a suicide car bomb attack.

Last November, Al-Nusra and ISIS had temporarily agreed to work together.

Thursday’s video comes two days after ISIS released another gruesome video showing it killing 16 "spies" in Nineveh province in northern Iraq.

The terrorists killed four men with a rocket-propelled grenade fired at a car, five by drowning in a metal cage, and seven by wrapping explosive cord around their necks and detonating it.

Since taking over swathes of land in Iraq and Syria, ISIS has executed hundreds of people by gunfire and dozens by beheading, while other victims have been stoned to death or thrown from buildings -- much of it documented in videos and photos.