Iraqi troops battled Islamic State extremists south of the militant-held city of Tikrit on Tuesday, but their offensive to retake Saddam Hussein's hometown is making little headway as forces are being slowed down by roadside bombs, local officials say.

The Iraqi troops are being supported by Iranian-backed Shiite militias and advisers, Sunni tribal fighters and Quds Force of Iran’s elite Revolutionary Guard.

Iran's semi-official Fars news agency reported Monday that Iranian Gen. Ghasem Soleimani, the commander of the elite Revolutionary Guard's Quds Force, was taking part in the offensive. Soleimani has emerged as the chief tactician in Iraq's fight against the Sunni militants.

Fierce clashes were under way mainly outside the town of al-Dour, south of Tikrit, while government troops were shelling militant bases inside the city, the officials said Tuesday. Tikrit fell to the Islamic State last summer and the offensive to retake it began Monday.

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

Ibrahim offered no specifics, though previous reports suggest extremists of 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 on with truck bomb on the outskirts of nearby Samarra targeting Iraqi forces and Shiite militiamen, identifying him by the nom de guerre of Abu Dawoud al-Amriki without elaborating.

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

The officials spoke anonymously to The Associated Press as they were not authorized to brief media.

Retired Maj. Gen. Bob Scales, who spoke to a source in U.S. Central Command Monday, told Fox News that the source said the U.S. military was caught off guard by Iraq’s decision to target Tikrit.

“They told us we’re gonna go to Mosul. Instead, they do a head-fake and they go to Tikrit without us,” Scales said.

Tikrit, the provincial capital of Salahuddin province, is located 80 miles north of Baghdad. It was taken by the Islamic State group along with the second-largest city in the north, Mosul, during last year's fighting.

Gen. Abdul-Wahab al-Saadi, the military commander of the Salahuddin region, told Iraqi state TV that troops would need some time to enter the city of Tikrit.

Iraqi and U.S. officials said Monday that the U.S.-led coalition was not involved in the Tikrit operation and had not been asked to carry out airstrikes.

Scales said he believes the U.S. is downplaying the mission for that reason.

“I think the Iraqis are saying to us: OK, thank you United States, we appreciate your advice and support mission, but we’re going to do this one on our own with Iran,” Scales told Fox News.

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 in the north and west during an onslaught last summer. Previous attempts to capture Tikrit all failed because of tough resistance from the militants.

“It’s very important because they made two runs at Tikrit over the last year, year and a half, and failed,” Scales said. “They are going in very heavy here – 25,000 people – if they fail again against a defensive force of only 1,500 people, they will never continue the fight up the Euphrates to Mosul -- which is their key objective. That’s the heartland of ISIS in Iraq.”

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.

The Associated Press contributed to this report.