BAGHDAD -- The Iraqi army on Saturday drove Islamic extremists from the center of a major city in central Iraq, for the first time mounting a concerted assault against insurgents who had charged to within 50 miles of Baghdad.
Independent sources, including local officials and witnesses, confirmed that an Iraqi army counteroffensive had driven militants of the Islamic State of Iraq and the Syria, or ISIS, from the center of Tikrit, including from government buildings as well as from major roads and other positions throughout the city.
But fighting was still continuing, with Iraqi warplanes bombing targets inside the city late in the afternoon.
Tikrit, the birthplace of Saddam Hussein, with a largely Sunni population of 250,000, is in the Tigris River valley, 100 miles north of Baghdad. It has long been a stronghold of anti-government Sunnis in Iraq, and losing it would sever the insurgents' lines of communication to Mosul and Syria. It could also strand some of their fighters in pockets south of Tikrit.
Some Iraqi military analysts said they thought it was no coincidence that the army's counteroffensive was launched now, with 180 of the 300 U.S. advisers ordered to Iraq by President Barack Obama arriving over the past three days, but Iraqi officials denied that there was any American role.
If the advances by the Iraqi army are sustained, and even built upon, it would provide a much needed morale boost for an army that lost as much as a fourth of its soldiers and equipment when ISIS overran Mosul, and has lurched from one embarrassment to another since then. It has given up the oil-rich city of Kirkuk to Kurdish forces. And it has lost all of its border crossing points with Syria and Jordan to the militants, ceding to them control of most of four major provinces extending more than 200 miles from north to south.
A spokesman for the Iraqi military, Gen. Qassim Atta, claimed that ISIS militants were withdrawing and that they had buried their dead on the grounds of a former Hussein palace in Tikrit. "Reports and surveillance show that ISIS leaders have ordered a retreat," he said.
Gen. Atta also said the government forces had killed Abu Abdul Hadi Baqiya, the ISIS commander for northern Tikrit.
Reports from medical officials at the provincial hospital in Tikrit said that ISIS had begun evacuating its wounded from the site on Friday.
ISIS, however, claimed a victory over government forces at the southern gates of the city, in postings on Twitter accounts associated with the extremists.
In other clashes, like the battle for control of the important Baiji refinery in Salahuddin province, both sides have sometimes traded control of contested territory on an almost daily basis.
"The signs of success are clear, and there is a very big change in the performance of the security forces, and now we can say the initiative is in the hands of the Iraqi forces," said Ahmed al-Sheraifi, a former air force pilot and now a professor at Baghdad University.
Mr. Sheraifi attributed the improved performance to intelligence support contributed by U.S. drones, which have begun flying over Iraq, as well as to American advice on tactics. "The security forces began relying on their airborne division, and this is a trademark of U.S. tactics," he said.
The Iraqi army's counteroffensive began Thursday with an airborne assault on the campus of Salahuddin University, which drove the militants out of that site. Early Saturday morning, after two days of fighting around the university, in downtown Tikrit, the Iraqi army launched a three-pronged attack, with a large body of ground troops driving into the city from the south and east, joined by troops garrisoned in Camp Speicher, north of the city, an Iraqi air force training base that never fell to the insurgents.