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Volunteer Shiite fighters, known as the Popular Mobilisation units, who support the Iraqi government forces in the combat against the Islamic State (IS) group fire a Howitzer artillery canon in the village of Awaynat near the city of Tikrit on February 28, 2015. Government forces have attempted and failed several times to wrest back Tikrit -- the hometown of former president Saddam Hussein -- since losing it to IS in June 2014. AFP PHOTO/AHMAD AL-RUBAYE / AFP / AHMAD AL-RUBAYE AND - (Photo credit should read AHMAD AL-RUBAYE/AFP/Getty Images)

Volunteer Shiite fighters, known as the Popular Mobilisation units, who support the Iraqi government forces in the combat against the Islamic State (IS) group fire a Howitzer artillery canon in the village of Awaynat near the city of Tikrit on February 28, 2015. Government forces have attempted and failed several times to wrest back Tikrit -- the hometown of former president Saddam Hussein -- since losing it to IS in June 2014. AFP PHOTO/AHMAD AL-RUBAYE / AFP / AHMAD AL-RUBAYE AND -        (Photo credit should read AHMAD AL-RUBAYE/AFP/Getty Images)