The horrific acts being carried out in the Syrian civil war know no bounds. The war has seen more than 120,000 people killed in just under three years, with many mass executions documented, as well as on thousands of women and girls, and torture of prisoners, sometimes ending in their death.

A new book currently on sale documents close to 3,000 victims of execution, who were killed by forces loyal to Syrian strongman Bashar al-Assad, according to its authors. Videos show entire families, from babies to old grandparents, who had their throats slashed with knives.

But Islamist rebels are also involved in mass atrocities. Over the last few months, pictures have been published showing rebels literally slaughtering Assad's soldiers. They even go a step further, often beheading their victims and showing the head to the camera, as a sign of victory over the Alawites and Shiites.

In a newly published video, made by an organization identified with Al Qaeda, terrorists proudly demonstrate what they call "the fastest way to behead Alawites."

A captive is seen kneeling, his hands tied behind his body and his eyes covered with cloth. In front of him is a pickup truck equipped with a heavy machine gun. A member of the Islamist organization points the gun to the victim's head and fires, as cries of "Allahu akbar" are heard.

The latest footage comes on the heels of a call by the Islamic State of Iraq and al-Shams (ISIS) for the systematic killing of Shia Muslims. ISIS is one of two Al Qaeda-linked groups operating in Syria, and those comments among others illustrate the sectarian nature of the civil war, as the mainly Sunni rebels seek to oust the regime of Bashar al-Assad, who is a member of the Alawite offshoot of Shia Islam.