TIKRIT, Iraq — Iraqi troops clashed along two fronts with Islamic State militants in Tikrit on Thursday as rockets and mortars echoed across Saddam Hussein's hometown a day after soldiers and allied Shiite militiamen swept into this Sunni city north of Baghdad.
Recapturing Tikrit is seen as a key step toward rolling back the gains of the extremist Islamic State group, which seized much of northern and western Iraq in a blitz last summer and now controls about a third of both Iraq and Syria.
The offensive also will serve as a major crucible for Iraqi forces, which collapsed under the extremists' initial offensive last year and now face one of the Sunni militant group's biggest strongholds.
Iraqi forces entered Tikrit for the first time on Wednesday from the north and south. On Thursday, they were fighting their way through the city and expected to reach the center within three to four days, according to Lt. General Abdul-Wahab al-Saadi, the commander of the Tikrit operation.
The IS militants were trying to repel the Iraqi forces with snipers, suicide car bombs, heavy machine guns and mortars, said al-Saadi, speaking to The Associated Press at the front-lines.
Tikrit, the capital of Salahuddin province, sits on the Tigris River about 130 kilometers (80 miles) north of Baghdad. Several of Saddam's palaces remain there, and supporters of the deceased dictator are believed to have played a key role in the Islamic State group's seizure of the city last year.
Iraqi Defense Minister Khaled al-Obeidi, who was also at the front-line on Thursday, told the AP that the operation to retake Tikrit is "essential to opening a corridor for security forces to move from the south to Mosul," he said, referring to Iraq's second-largest city and the militants' biggest stronghold.
He described the operation as "100% Iraqi, from the air and ground."
When the Islamic State last year swept into Mosul, the U.S.-trained Iraqi military crumbled and the militants seized tanks, missile launchers and ammunition, steamrolling across northern Iraq. The CIA estimates the Sunni militant group has access to between 20,000 and 31,000 fighters in Iraq and Syria. Military officials believe there may about 150 foreign fighters with the IS inside Tikrit, including fighters from Chechnya and the Arab Gulf countries.
Iraqi officials now say that at least 30,000 men — including the military, militias, Sunni tribes and police — are fighting to capture Tikrit.
U.S. Gen. Martin Dempsey, the Joint Chiefs chairman, said Wednesday that at least 20,000 militiamen are taking part in the Tikrit fighting.
On Thursday, militiamen were heard intercepting IS walkie-talkie signals, listening to the militants' call for reinforcements and ordering mortar fire on the soldiers as they closed in. Along the route between Salahuddin's command center and the battlefield, charred remains of tankers and cars used by suicide bombers litter the roads, and homes bear signs of months of war, damaged by bombs and bullets.
Military officials told the AP they are advancing with caution in an effort to limit damage to the city's infrastructure, so that residents can return quickly once Tikrit is retaken. A satellite image of Tikrit, released last month by the United Nations, showed that at least 536 buildings in Tikrit have been affected by fighting, with at least 137 completely destroyed and 241 severely damaged.
Earlier Thursday, al-Obeidi visited troops and met with senior military commanders of the Tikrit operation as well as Iranian Maj. Gen. Qassem Soleimani, commander of the Quds Force, an elite unit of Iran's powerful Revolutionary Guard. Soleimani and other Iranian advisers have played a key role in Iraq in pushing the Islamic State back in recent months.
The overt Iranian role and the prominence of Shiite militias in the campaign have raised fears of possible sectarian cleansing should Tikrit, an overwhelmingly Sunni city, fall to the government troops.
The United States, which spent billions of dollars training and equipping Iraq's army during its eight-year intervention, has said its allied coalition carrying out airstrikes targeting the extremists has not been involved in the ongoing Tikrit offensive.
In November, President Barack Obama authorized the deployment of up to 1,500 more American troops to bolster Iraqi forces, which could more than double the total number of U.S. forces to 3,100. None has a combat role.
Iraq's Prime Minister Haider al-Abadi has appealed for more aid for his country's beleaguered ground forces, although the U.S. spent billions of dollars training and equipping Iraq's army during its eight-year occupation.
The growing Iraqi impatience in many ways stems from concerns about the speed and success of the Islamic State's advance, and the Baghdad government's inexperience in handling a security crisis of this magnitude. Until recently, Iraqi security forces were focused on protecting themselves and the population against insurgent bombings and other attacks, not on repelling an advancing force or retaking areas seized by the militants.
By contrast, Islamic State militants appear to operate in a fluid, decentralized command structure that has enabled them to adapt quickly and more nimbly to the changing environment amid airstrikes and Iraqi and Kurdish ground offensives.