Iraqi troops and Shia militias have battled against Islamic State south of the militant-held city of Tikrit, though roadside bombs and suicide attacks slowed their advance on Saddam Hussein's hometown.
The battle for Tikrit, a strategic city along the Tigris River, is likely to be won or lost on allied Iraqi forces' ability to counter the extremists' bombs. Such explosives were a mainstay of al Qaida in Iraq, the IS group's predecessor, as it fought American forces following their 2003 invasion of the country.
Reports suggest extremists from IS, which holds both a third of Iraq and neighbouring 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 IS has said an American jihadi carried out a suicide attack on the outskirts of nearby Samarra targeting Iraqi forces and Shia militiamen.
On Tuesday afternoon, a suicide bomber drove a military vehicle into a checkpoint manned by government forces and Shia fighters south of Tikrit, killing four troops and wounding 12, a police officer and medical official said.
Tuesday marked the second day of the Iraqi advance on Tikrit, with its soldiers supported by Iranian-backed Shia militias and advisers, along with some Sunni tribal fighters who reject IS. Already, Iran's semi-official Fars news agency has reported that Iranian General 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 on 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.
Past attempts to retake Tikrit have failed as Iraq struggles with its armed forces, which collapsed last summer. The military 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.
Tikrit, the provincial capital of Salahuddin province, is located 80 miles (130 kilometres) north of Baghdad. It was taken by IS along with the country's second-largest city, Mosul, during the militants' lightning advance last year across the north. Retaking Tikrit will help Iraqi forces secure a major supply link for any future operation to seize Mosul.
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 the Americans have cautioned that if the Iraqis are not ready, the offensive could be delayed. On Monday, Iraqi and US officials said the US-led coalition was not involved in the Tikrit operation and had not been asked to carry out air strikes.
Iraq is bitterly split between minority Sunnis, who were an important base of support for Saddam, and the Shia majority. Since Saddam was toppled and later executed, the Sunni minority has felt increasingly marginalised by the Shia-led government in Baghdad. In 2006, long-running tensions boiled over into sectarian violence that claimed tens of thousands of lives.
IS tapped into that Sunni resentment, though prime minister Haider Abadi, a Shia, 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 to Saddam.