By SINAN SALAHEDDIN and SAMEER N. YACOUB, Associated Press

BAGHDAD (AP) — Iraqi troops and Shiite militias battled the Islamic State group on Tuesday on the outskirts of militant-held Tikrit, unable to advance further on Saddam Hussein's hometown as roadside mines and suicide attacks slowed their progress.

Soldiers found some 100 mines and bombs scattered along an 8-kilometer (5-mile) stretch of road on the way to this strategic city on the Tigris River, Salahuddin deputy governor Ammar Hikmat said.

The discovery underlined how the battle likely will pivot on allied Iraqi forces' ability to counter such weapons, a mainstay of al-Qaida in Iraq, the Islamic State group's predecessor, as it fought American forces following their 2003 invasion of the country.

The bombs are "the main obstacle in the way of the attacking forces, which have to wait for bomb experts or to go around the area," Hikmat told The Associated Press. "And this costs time."

Extremists from the Islamic State group, which holds both a third of Iraq and neighboring Syria in its self-declared caliphate, have littered major roadways and routes with mines. Such mines allow the extremists to slow any ground advance and require painstaking clearing operations before troops can safely move through.

Suicide bombings also aid the militants in weakening Iraqi forces and have been used extensively in its failed campaign for the Syrian border town of Kobani. Already, a militant website affiliated with the Islamic State group has said an American jihadi carried out a suicide attack with a truck bomb on the outskirts of the nearby city of Samarra, targeting Iraqi forces and Shiite militiamen. The posting identified him by the nom de guerre of Abu Dawoud al-Amriki, without elaborating.

A suicide bomber also drove a military vehicle Tuesday afternoon into a checkpoint manned by government forces and Shiite fighters south of Tikrit, killing four troops and wounding 12, a police officer and a medical official said.

Tuesday marked the second day of the Iraqi advance on Tikrit, with soldiers supported by Iranian-backed Shiite militias and advisers, along with some Sunni tribal fighters who reject the Islamic State group. Hikmat estimated the Iraqi force besieging Tikrit at some 25,000 people. Iran's semi-official Fars news agency has reported that Iranian Gen. Ghasem Soleimani, the commander of the country's elite Revolutionary Guard's Quds Force, was taking part in the offensive.

Government forces, however, made little headway Tuesday, two local officials said. They said fierce clashes struck mainly outside the town of al-Dour, south of Tikrit, while government troops shelled militant bases inside the city. Those officials spoke on condition of anonymity as they were not authorized to brief journalists.

Brig. Gen. Saad Maan Ibrahim, an Interior Ministry spokesman, said explosive experts had disabled "so many bombs and car bombs."

"Tikrit has been besieged from three directions, from the north, west and south, but what has remained only from the eastern side," Ibrahim said. "The explosive experts were able to tackle so many bombs and car bombs."

Hikmat said the offensive had killed and wounded "dozens" of Islamic State extremists, but that the attacking forces also have been killed. Authorities in Baghdad offered no immediate casualty figures.

Past attempts to retake Tikrit have failed, as Iraq struggles with a military that collapsed last summer during the Islamic State militants' lightning offensive. The Tikrit operation is seen as a litmus test for the capability of Iraqi troops to dislodge the militants from major cities they conquered in the country's Sunni heartland.

Retaking Tikrit, the provincial capital of Salahuddin province, some 130 kilometers (80 miles) north of Baghdad, would help Iraqi forces secure a major supply link for any future operation to capture Mosul, the country's second-largest city.

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 the Americans have cautioned that if the Iraqis aren't ready, the offensive could be delayed.

On Monday, Iraqi and U.S. officials said the U.S.-led coalition was not involved in the Tikrit operation and had not been asked to carry out airstrikes. Overall, coalition airstrikes have killed more than 8,500 Islamic State fighters since its campaign began in August, said Army Gen. Lloyd Austin, the commander of U.S. Central Command.

"The fact is that (the group) can no longer do what (it) did at the outset, which is to seize and to hold new territory," Austin said.

As the Tikrit battle rages, Iraq remains bitterly split between minority Sunnis, who were an important base of support for Saddam, and the Shiite majority. Since Saddam was toppled and later executed, the Sunni minority has felt increasingly marginalized by the Shiite-led government in Baghdad. In 2006, long-running tensions boiled over into sectarian violence that claimed tens of thousands of lives.

The Islamic State group tapped into that Sunni resentment, though Prime Minister Haider al-Abadi, a Shiite, has offered an amnesty for insurgents who abandon the extremists. His comments appeared to be targeting former members of Iraq's outlawed Baath party, loyalists of Saddam.

Later on Tuesday evening, a bomb exploded in a commercial street in southeastern Baghdad, killing three people and wounding nine, police and hospital officials said, speaking on condition of anonymity because they were not authorized to speak to the media.

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Associated Press writers Maamoun Youssef and Jon Gambrell in Cairo and Robert Burns in Washington contributed to this report.

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