Islamic State group jihadists beheaded 12 men from rival Syrian rebel movements accused of fighting against them, in a video released on Thursday.

It is the latest in a long series of mass beheadings by IS, and comes two days after the group released a video showing it killing 16 people in neighbouring 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 IS's main jihadist rival in the country.

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

On Tuesday IS released another gruesome video showing it killing 16 "spies" in Nineveh province in northern Iraq.

The militants 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.

Such videos serve multiple purposes for IS, drawing international attention to the group, aiding recruitment and inspiring fear in its opponents.

IS 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.