The move is a major step in a campaign to reclaim a large swath of territory in northern Iraq controlled by the militants.
But hours into the operation, a key test for the embattled Iraqi army, the military said it had still not entered the city, indicating a long battle lies ahead.
Tikrit, the provincial capital of Salahuddin province, 80 miles north of Baghdad, fell into IS’s hands last summer, along with the country’s second-largest city, Mosul, and other areas in the country’s Sunni heartland, after the collapse of national security forces. Tikrit is one of the largest cities held by IS forces and sits on the road to Mosul.
Security forces have so far been unable to retake Tikrit, but momentum has begun to shift since soldiers, backed by air strikes from a United States-led coalition, took back the nearby refinery town of Beiji in November. Any operation to take Mosul would require Iraq to seize Tikrit first because of its strategic location for military enforcements.
US military officials have said a co-ordinated military mission to retake Mosul is likely to begin in April or May and involve up to 25,000 Iraqi troops. But they have cautioned that if the Iraqis are not ready, the timing could be delayed.
Past attempts to retake Tikrit have failed, and Iraqi authorities say they have not set a date to launch a major operation to recapture Mosul. Heavy fighting between IS and Kurdish forces is taking place only outside the city.
Al-Iraqiya television said the forces were attacking Tikrit from different directions, backed by artillery and air strikes by Iraqi fighter jets. It said the militants had been dislodged from some areas outside the city. Several hours into the operation, it gave no details.
The military commander of Salahuddin region, General Abdul-Wahab Saadi, told state TV the operation was “going on as planned”, with fighting taking place outside Tikrit, mainly on its eastern side.
“Until this moment, we have not entered the city,” Gen Saadi said. “God willing, we will enter, but we need some time as planned,” he said, adding that there was no timeframe for the operations.
“God willing, victory will be achieved and Salahuddin will be turned into a grave for all terrorist groups,” he added.
Tikrit is an important test case for Iraq’s Shia-led government, which is trying to reassert authority over the divided country. IS fighters have a strong presence in the city and are expected to put up fierce resistance.
While TV said Shia and Sunni tribal fighters were co-operating in the offensive, Tikrit is a key Sunni stronghold, and the presence of Shia forces could prompt a backlash among Sunnis.
Iraq is bitterly split between minority Sunnis, who were an important base of support for Saddam, and the Shia majority. Since Saddam was toppled in a US-led invasion in 2003, the Sunni minority has felt increasingly marginalised by the Shia-led government in Baghdad.