A member of the Iraqi counterterrorism forces waves the national flag after securing a checkpoint from Sunni militants west of Mosul. (Ahmad Al-Rubaye/AFP/Getty Images)

The Iraqi army launched a fresh offensive Tuesday to recapture the central city of Tikrit, seeking to build on the success of U.S.-backed government forces in seizing Mosul Dam, their first significant defeat of Islamic State militants this year.

Initial reports suggested, however, that the offensive had stalled in the face of stiff resistance from the militant fighters, who swept into the home town of former dictator Saddam Hussein in June with significant support from its residents, who are overwhelmingly Sunni.

American airstrikes played a decisive role in routing the extremist fighters from Mosul Dam, which was fully under government control Tuesday. The recapture of the dam in northern Iraq came after a two-day battle waged on the ground by Iraqi special-operations forces and Kurdish pesh merga fighters, backed by American bombers, drones and fighter jets.

A total of 35 U.S. attacks on Islamic State positions destroyed more than 90 targets, Pentagon officials said, enabling the joint Kurdish-Iraqi force to move swiftly into the vital facility. The dam provides electricity to much of northern Iraq, and sabotage of it by the militants could have triggered catastrophic flooding.

The U.S. intervention has been confined to areas bordering the semiautonomous Iraqi region of Kurdistan that were overrun by the Islamic State in the first week of August, prompting President Obama to authorize airstrikes to halt the militant advance.

But the intervention appeared to have energized Iraqi forces further south of the vast stretch of territory across Iraq and Syria conquered by the Sunni extremist Islamic State over the past year.

Iraqi security forces backed by Shiite militias pushed toward Tikrit in a three-pronged offensive starting after dawn Tuesday, Iraqi officials said.

Fighting continued late into the night Tuesday, according to a resident contacted by telephone who reported explosions and gunfire in the vicinity of the provincial council building, the headquarters of the local government. The resident spoke on the condition of anonymity for security reasons.

Capt. Firas Sami of the Iraqi army said government forces were close to regaining control of the headquarters. Fighters with the Shiite Asaib Ahl al-Haq militia joined the offensive alongside the Iraqi army, Sami said. Their presence underscored the complications that would arise if the U.S. military were to extend its help to government forces battling the Islamic State further to the south.

The Iranian-backed Asaib Ahl al-Haq was responsible for multiple bombings against U.S. forces during their eight-year presence in Iraq and in the last years were identified by American commanders as a bigger threat to the lives of U.S. troops than al-Qaeda.

Tikrit is a Sunni-dominant town, regarded as a stronghold of the militants, and two previous attempts by the Iraqi army to recapture it failed dismally.

An Iraqi army spokesman, Lt. Gen. Qassim al-Moussawi, told reporters in Baghdad that he expected this latest push to be “slow and gradual.”

“There are still a lot of challenges and difficulties ahead of us,” he said at a briefing broadcast on state TV.

“The war needs time, but we are determined to annihilate the Islamic State [militants] and to liberate all the areas they occupy — even if we suffer heavy causalities, because we have no other choice,” he said.

Tikrit was overrun by the Islamic State in early June after its push into the northern city of Mosul sent Iraqi army soldiers fleeing, discarding their uniforms as they ran. The militants’ sweep took them to within 60 miles of Baghdad, and although the Iraqi security forces have since managed to halt their advance, they have had few successes in reversing their gains.