The Islamic State, which is rapidly moving towards establishing itself as a 'sovereign state,' has massacred over 300 Sunni Albu Nimr tribesmen in the north west of Iraq.
Justifying the mass execution, ISIS said the killings were carried out as per the principles of Quran.
The execution is said to have been carried out by the 'Islamic State Counter-Terrorism Implementation Task Force.
Reports have also indicated the Anbar killings were undertaken by the Islamic State Counter-Terrorism Implementation Task Force.
The bodies of 150 Albu Nimr tribesmen were recovered from a mass grave near Ramadi in Anbar province, while another 48 bodies were found near Heet, a Daily Mail report said.
ISIS warned that a similar fate awaited all those who supported the enemy, which includes the Iraqi Shia army and the US forces.
Reuters reported the victims, aged between 18 and 55, were members of the police force or an anti-Islamic State Sunni force called Sahwa (Awakening). The ISIS supporters claimed on online forums that the 'Abu Nimr' was working for Rafidah, or Shia'a Muslims.
Twitter handle SaudiWitness @SaudiWitness said several 'Mujahideen' from the same clan witnessed the execution of the tribesmen.
The ISIS supporters accused the tribesmen, who joined the Iraqi army, of losing 'Loyalty to Islam'.
ISIS leader Abu Wahab, who orchestrated the massacre, gained notoriety following the filmed murder of a group of Syrian lorry drivers in Iraq last summer. His band of militants, known as the 'Al Anbar Lions,' has been responsible for several killings in the region.