Iraqi troops and Shia militias have battled against Islamic State (IS) 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.

Yesterday 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.

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.

Reports in Iran said Iranian General Ghasem Soleimani, the commander of the country's elite Revolutionary Guard's Quds Force, was taking part in the offensive.

There were fierce clashes 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 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.