BAGHDAD -- Backed by allied Shiite and Sunni fighters, Iraqi security forces on Monday began a large-scale military operation to recapture Saddam Hussein's hometown from the Islamic State of Iraq and Syria (ISIS), state TV said, 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 still hadn't entered the city of Tikrit, indicating a long battle lies ahead.

Tikrit, the provincial capital for Salauhddin province, 80 miles north of Baghdad, fell into the hands of ISIS last summer along with the country's second-largest city of 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 ISIS 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 airstrikes from a U.S.-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.

U.S. military officials have said a coordinated military mission to retake Mosul will likely begin in April or May and involve up to 25,000 Iraqi troops. But they have cautioned that if the Iraqis aren't ready, the timing could be delayed.

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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 ISIS and Kurdish forces is taking place only outside the city.

Al-Iraqiya television said that the forces were attacking Tikrit from different directions, backed by artillery and airstrikes by Iraqi fighter jets. It said the militants were dislodged from some areas outside the city. Several hours into the operation, it gave no details.

The military commander of Salahuddin region, Gen. Abdul-Wahab al-Saadi, told the 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," al-Saadi said. "God willing, we will enter, but we need some time as planned," he said, adding that there is no timeframe for the operations.

"God willing, victory will be achieved and Salahuddin will be turned into a grave for all terrorist groups," he said.

Tikrit is an important test case for Iraq's Shiite-led government, which is trying to reassert authority over the divided country. Islamic State fighters have a strong presence in the city and are expected to put up fierce resistance.

Iraq is bitterly split between minority Sunnis, who were an important base of support for Saddam, and the Shiite majority. Since Saddam was toppled in a U.S.-led invasion in 2003, the Sunni minority has felt increasingly marginalized by the Shiite-led government in Baghdad, and in 2006 long-running tensions boiled over into sectarian violence that claimed tens of thousands of lives.

While the TV said Shiite and Sunni tribal fighters were cooperating in Monday's offensive, Tikrit is an important Sunni stronghold, and the presence of Shiite forces risks could prompt a backlash among Sunnis. The Iraqi military is heavily dependent on Shiite militias that have been accused of abusing Sunni communities elsewhere in Iraq.

Hours ahead of the operation, Prime Minister Haider al-Abadi, a Shiite, called on Sunni tribal fighters to abandon the Islamic State extremist group, offering what he described as "the last chance" and promising them a pardon.

"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, 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.

His comments appeared to be targeting former members of Iraq's outlawed Baath party, loyalists to Saddam, who joined ISIS during its offensive, as well as other Sunnis who were dissatisfied with Baghdad's Shiite-led government.

Saddam, whose Sunni-dominated government ruled the country for some two decades, was executed after his ouster. Tikrit frequently saw attacks on U.S. forces during the American occupation of the country.