WASHINGTON » At Iraq’s request, the U.S. began airstrikes in Tikrit on Wednesday in support of a stalled Iraqi ground offensive to retake the city from Islamic State fighters. The bombing marked a significant expansion of the U.S. military role in Iraq.

"These strikes are intended to destroy ISIL strongholds with precision, thereby saving innocent Iraqi lives while minimizing" unintended damage to civilian structures, Lt. Gen. James L. Terry, the commander of the U.S.-led campaign to defeat the Islamic State group, said in a written statement.

"This will further enable Iraqi forces under Iraqi command to maneuver and defeat ISIL in the vicinity of Tikrit," Terry said, using an acronym for the Islamic State group.

Tikrit is deemed an important test of the ability of Iraq, with coalition support, to retake ground it ceded to the Islamic State last year. The U.S. initially did not provide air support in Tikrit because Baghdad pointedly chose instead to partner with Iran in a battle it predicted would yield a quick victory. In recent days, however, the Pentagon has called the Iraqi offensive "stalled."

An Associated Press correspondent in Tikrit reported hearing warplanes overhead late Wednesday, followed by multiple explosions. An Iraqi commander in the city told the AP that a warehouse used to store Islamic State weapons was bombed by a U.S. plane, and a U.S. official in Washington confirmed that arms warehouses were among the targets. The Iraqi commander spoke on condition of anonymity because he was not authorized to discuss details of the airstrikes.

The Washington official, who also spoke on condition of anonymity in order to discuss military details, said there were no more than one dozen airstrikes Wednesday, and said some were conducted by U.S. allies. The official had no details on the extent of allied participation, including which countries launched airstrikes.

The official said Wednesday’s attacks were the first in a series that would be carried out in the days to come as the coalition coordinates with Iraqi ground troops who have encircled Tikrit but not penetrated deeply into the city.

The battle for Tikrit is widely seen as a step toward the more difficult and potentially decisive battle to regain control of the larger city of Mosul.

In an address to the nation Wednesday evening, Prime Minister Haider al-Abadi predicted success in Tikrit but did not say the U.S. was providing airstrikes.

"We have started the final phase of the operation in Tikrit," he said. "You will liberate your ground, not anyone but you," he said in a speech to the Iraqi people.

Al-Abadi praised all the groups involved in the battle against the Islamic State group, including the so-called Popular Mobilization Forces, which the U.S. calls Iranian-backed Shiite militias, and well as the Sunni tribes and coalition forces. But he fell short of confirming that the coalition is playing a direct role in Tikrit.

U.S. airstrikes in Tikrit raise highly sensitive questions about participating in an Iraqi campaign that has been spearheaded by Iraqi Shiite militias trained and equipped by Iran, a U.S. adversary.

Iran has provided artillery and other weaponry for the Tikrit battle, and senior Iranian advisers have helped Iraq coordinate the offensive. U.S. officials have estimated that two-thirds of the ground troops involved in the offensive are Shiite militias; the others are combinations of regular Iraqi army soldiers and Sunni tribal fighters.

In his statement Wednesday, Terry said the U.S. airstrikes were aimed at energizing the Iraqis.

"Renewed efforts on the ground supported by the coalition are aimed at dislodging ISIL fighting elements from Tikrit and once again placing the town under (Iraqi) control," Terry said.

The U.S. has hundreds of military advisers in Iraq helping its security forces plan operations against the Islamic State, which occupies large chunks of northern and western Iraq. But the U.S. has said it is not coordinating any military actions with the Iranians.

Col. Steve Warren, a Pentagon spokesman, said that at Baghdad’s request the U.S. began aerial surveillance over Tikrit in recent days and is sharing the collected intelligence with the Iraqi government.

The U.S.-led air campaign, launched in August and joined by several European allies, has allowed Iraqi forces to halt the IS group’s advance and claw back some of the territory militants seized last summer.

But the growing Iranian presence on the ground has complicated the mission, with Washington refusing to work directly with a country it views as a regional menace, even though it is currently embroiled with Iran in sensitive negotiations over a nuclear deal.

The prominent role of the Shiite militias in the fight to retake Tikrit and other parts of Iraq’s Sunni heartland has also raised concerns that the offensive could deepen the country’s sectarian divide and drive Sunnis into the arms of the Islamic State group.

Hadi al-Amiri, leader of the Badr Organization and a commander of Iraq’s Shiite militias, told reporters in Samarra: "If we need them (the U.S.-led coalition ) we will tell them we need them. But we don’t need the coalition. We have surveillance planes over our heads already. The participation of U.S. planes hinders out operations. … If we need it, we’ll tell our government what we need."

He claimed the militias, the overwhelming majority of which are made up of Shiite fighters, have their own surveillance drones. "We buy them anywhere," he said. "We have our own … controlled by Iraqis."

A series of U.S. airstrikes north of Tikrit, in the vicinity of Beiji, in recent weeks has had the indirect benefit of tying down Islamic State forces that might otherwise be operating in defense of Tikrit. On Wednesday, for example, the U.S. military said it had conducted five airstrikes Tuesday near Beiji, home of a major oil refinery that IS has sought to capture. That bombing targeted IS combat units and destroyed what the U.S. called an IS "fighting position," as well as an IS armored vehicle.

Associated Press writers Qassim Abdul-Zahra in Tikrit, Iraq, and Lolita C. Baldor in Washington contributed to this report.