U.S. warplanes began conducting airstrikes in Tikrit on Wednesday after the Iraqi government requested assistance with a stalled offensive to retake the militant-held city, officials said.
"I can confirm that the government of Iraq has requested coalition support for operations in Tikrit," Pentagon spokesman Col. Steve Warren said in a statement. "Operations are ongoing."
Another defense official, speaking on the condition of anonymity to discuss details of the campaign, said American planes were taking part in the strikes.
The United States had hesitated to be drawn into the Iraqi government's drive to take back the mostly Sunni Muslim city from the extremist group Islamic State because of the involvement of thousands of Shiite militiamen and the presence of at least one major commander from Shiite Iran.
Concern has been raised about the possibility of reprisal attacks for Islamic State's slayings of hundreds of Shiite soldiers and policemen last summer, when the group seized large parts of northern and western Iraq. Shiite militias have been seen demolishing and setting fire to the homes of Sunni civilians in other towns wrested from Islamic State control.
"These strikes are intended to destroy [Islamic State] strongholds with precision, thereby saving innocent Iraqi lives while minimizing collateral damage to infrastructure," Lt. Gen. James L. Terry, who commands the coalition of countries conducting airstrikes against the militants.
"This will further enable Iraqi forces under Iraqi command to maneuver and defeat ISIL in the vicinity of Tikrit," he said, using one of several acronyms for Islamic State.
American and allied planes are also providing overhead video feeds of the fighting to the Iraqi government, and coalition advisors are in Iraqi military headquarters providing advice and assistance, officials said.
As many as 30,000 Iraqi government troops and Iran-backed militiamen, supported by a contingent of Sunni tribesmen, are taking part in the campaign in Tikrit, the home town of late strongman Saddam Hussein.
Pro-government forces have faced sniper fire and hundreds of explosive devices spread over the city and its surrounding villages, according to state media accounts. But officials have been pressed to explain why the city has not been retaken weeks after the campaign began.
State Department spokeswoman Jen Psaki said in a statement that the U.S. and Iraqi governments "will continue to work together on our shared goal of defeating [Islamic State] and training a professional national security force that can protect all the Iraqi people against extremist threats."