The AP reports that Al-Iraqiya television confirmed the operation—"backed by artillery and airstrikes by Iraqi fighter jets"—and claimed some militants had already been driven out of the area.
ISIS took control the city of Tikrit—best known as Saddam Hussein's hometown—last summer. According to the AP, it is one of the largest cities currently held by the terrorist group and a key area.
Prime Minister Haider al-al-Abadi reportedly asked ISIS members to abandon the cause before the offensive began, promising an official pardon in exchange.
"I call upon those who have been misled or committed a mistake to lay down arms and join their people and security forces in order to liberate their cities," al-Abadi said Sunday during a news conference in Samarra, 95 kilometers (60 miles) north of Baghdad.
Al-Abadi offered what he called "the last chance" for Sunni tribal fighters, promising them a pardon. "The city will soon return to its people," he added.