begins on march 15th. iraqi and other forces on the out skirts of tikrit advancing in the battle to retake the city. isis planted ieds on roads and bridges leading to the city. among the troops for the battle forty tikrit. senior correspondent frederik pleitgen has the latest. reporter: as the iraqi military moves forward and the heart of tikrit but makes advances in other place, it is the iranians who are the ones doing the heavy lifting and the ones making the main differences on the ground. now, they have never acknowledged they have combat forces in iraq however, they say they have some very high profile trainers and certainly people from the revolution area