Shi'ite fighters cover their ears as a rocket is launched during a clash with Islamic State militants in the town of al-Alam March 9, 2015.[Photo/Agencies]

The small Ajil oilfield is one of the most important financial resources to fund the IS militant group which extracts about 10, 000 barrels per day and convey them by some 300 tanker trucks to other areas in Nineveh province and IS-held areas in Syria.

The Iraqi security forces began their operation last week via five paths: two from Mkesheifa and Sur Shnas, south of Tikrit, two others from Udheim and Tuz Khurmato, east of Tikrit, and the last from the Speicher airbase, just north of Tikrit, according to security sources.

Large parts of Salahudin province have been under IS control since June 11, a day after bloody clashes broke out between Iraqi security forces and the group.

The IS has taken control of the country's northern city of Mosul and later seized swathes of territories in Nineveh and other predominantly Sunni provinces.