BAGHDAD - Backed by Iranian-supported Shiite militias, Iraqi forces launched a large-scale offensive Monday to retake Saddam Hussein's hometown from the Islamic State group, the first in a series of campaigns to try to reclaim large parts of northern Iraq from the Sunni extremists.

Previous attempts to capture the symbolic city have failed, and hours into Monday's operation, the military said it still hadn't entered Tikrit, indicating a long battle lies ahead. Retaking it will help Iraqi forces secure a major supply link for any future operation to retake Mosul, Iraq's second-largest city, which has been under militant rule since June.

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

Tikrit, the provincial capital of Salauhddin province, 130 kilometres north of Baghdad, fell to the Islamic State group last summer, along with Mosul and other areas in the country's Sunni heartland.

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

The U.S.-led coalition launching airstrikes targeting the Islamic State group was not involved in the Tikrit operation, Iraqi officials said. A Pentagon spokesperson, Army Col. Steve Warren, said the U.S. was alerted to the offensive before it started Monday but was not asked to provide air power.

"Right now, we are not providing any air power to support the Iraqi operation in the city of Tikrit," Warren told reporters in Washington. "We did note the Iraqi government's statements that they are emphasizing minimization of collateral damage, and we are continuing to monitor it."

Iraqi forces apparently have the help of Iranian Gen. Ghasem Soleimani, the commander of the elite Revolutionary Guard's Quds Force, who arrived two days ago, the Iranian semi-official Fars news agency reported.

The powerful general has emerged as the chief tactician in Iraq's fight against the Sunni militants, working on the front lines alongside dozens of advisers from his country's Revolutionary Guard to direct Shiite militiamen and government forces in the smallest details of battle.

Fars also reported drones were flying over Tikrit, without identifying whether they were Iranian or Iraqi.

The military commander of Salahuddin region, Gen. Abdul-Wahab al-Saadi, told state TV fighting was 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. . God willing, victory will be achieved and Salahuddin will be turned into a grave for all terrorist groups."

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.

Past attempts to retake Tikrit have failed, as Iraq struggles with its armed forces, which collapsed in the wake of the Islamic State group's offensive last summer.

The offensive comes as momentum has begun to shift since Iraqi soldiers, backed by airstrikes from the 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.

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 state TV said Shiite and Sunni tribal fighters were co-operating in Monday's offensive, Tikrit is an important Sunni stronghold, and the presence of Shiite forces 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 after the offensive began, the UN special envoy in Iraq appealed to warring groups to avoid attacking civilians.

"Military operations reinforced by international and Iraqi air support must be conducted with the utmost care to avoid civilian casualties, and with full respect for fundamental human rights principles and humanitarian law," Nickolay Mladenov said in a statement.

Monday afternoon, a gasoline tanker rigged with a bomb exploded as soldiers and Shiite militiamen tried to dismantle it in the village of al-Jalam south of Tikrit, killing seven troops and wounding 15, police and hospital officials said.

Al-Jalam, a farming area that has been a stronghold of Sunni militants, is located outside the Sunni city of Samarra, 95 kilometres north of Baghdad. All officials spoke on condition of anonymity because they were not authorized to speak to journalists.

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.

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"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.

His comments appeared to be targeting former members of Iraq's outlawed Baath Party, loyalists to Saddam, who joined the Islamic State group 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.