Three pictures published on a jihadist forum and pro-IS social media accounts show the execution of the men wearing orange jumpsuits.
Local residents confirmed to AFP that a mass execution had taken place on a large roundabout six kilometres (four miles) east of the city of Tikrit at around 3:30 pm (1230 GMT).
The first picture shows 11 men kneeling, heads bowed, with one black-clad and masked gunman behind each one and black IS flags in the background.
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The next picture shows the gunmen brandishing their handguns after the execution and in the third a small gathering of onlookers, including children, can be seen near 13 bodies, which have been dragged off the roundabout's central island and onto the road.
Residents said the roundabout is at an intersection for roads leading to Tikrit, Kirkuk and the town of Al-Alam.
They said the men who were executed were members of an anti-IS group of Sunni tribal fighters known as the Knights of Al-Alam who were captured by jihadists in Tikrit and Al-Alam around 10 days earlier.
The city of Tikrit has been under IS control since the beginning of the jihadists' major offensive in Iraq six months ago.
Iraqi government forces, backed by Shiite militia and in some areas by Sunni tribal fighters have inflicted a string of defeats on the jihadists in recent weeks.