The Islamic State jihadist group released a video on Monday in which four Sunni tribesmen from the Tikrit region are executed for allegedly collaborating with the government.
The video was released on the day some 30,000 government forces launched a huge operation to retake Tikrit, which the jihadists have controlled for almost nine months.
The video describes them as belonging to a tribal group named after a town just north of Tikrit.
"Elimination of a Sahwat al-Alam cell," reads a banner just before masked gunmen are seen shooting a bullet into the back of each victim's head.
IS released pictures of the execution of 13 alleged members of the group in December.
According to military sources, several units of Sunni tribal fighters were involved in the offensive launched on Monday, alongside the army, police, Shiite volunteers and militias.
Announcing the operation on Sunday, Prime Minister Haider al-Abadi urged all Tikrit resident to lay down their arms and turn against IS.