U.S. warplanes began striking Islamic State forces in and around the Iraqi city of Tikrit on Wednesday, drawing the United States directly into a battle that has pitted the militants against Iraqi forces dominated by Iranian-backed militias.

Pentagon officials said that the Iraqi government had requested the assistance as the fight for Tikrit stalled as it moved into its fourth week. They said initial targeting for the strikes will be aided by U.S.-led coalition surveillance aircraft that recently began flying over the city, 110 miles northwest of Baghdad.

The fight for Tikrit is considered a crucial test for larger future objectives, including Mosul, Iraq’s second-largest city, which has been the symbol and center of Islamic State power in Iraq since the militants took it last summer.

But the Tikrit operation is fraught with potential political and strategic complications for the Obama administration. The overwhelming presence of Shiite militias and volunteers armed and advised by Iran has given rise to fears that their victory would promote sectarian divisions and bloodletting in the majority-Sunni city. U.S. officials have estimated that these Shiite fighters outnumber official Iraqi security forces and Sunni tribal forces by about 5 to 1 in the battle.

Powerful Sunni Arab states that are part of the U.S.-led coalition against the Islamic State, including Saudi Arabia, have warned against the growing regional power of Shiite Iran. In Washington, some lawmakers have charged that Obama is allowing Iran to expand its influence in Iraq to avoid undermining separate nuclear negotiations with Tehran whose deadline is next week.

U.S.-led coalition warplanes launched their first airstrikes against Islamic State targets in Tikrit on Wednesday, officials said, to aid Iraqi forces fighting alongside Iran-backed Shiite militia on the ground. (Reuters)

But U.S. officials, insisting the two issues are not linked, have said that victory over the Islamic State has to be the first order of business in Iraq and have indicated that they welcomed Iranian assistance there. Although U.S. aircraft that have bombed other parts of Iraq have stayed away from the area until now to avoid appearing to be aiding the ­Iranian-backed forces, the battlefield stasis in Tikrit apparently forced a change of heart.

A Pentagon statement announcing the airstrikes repeatedly emphasized that they were “to support Iraqi Security Forces . . . after a request from the Iraqi Prime Minister, Haider al-Abadi.” Operations “to expel ISIL from the city,” it said, included “providing airstrikes, airborne intelligence capabilities, and advise and assist support to Iraqi Security Force headquarters elements.” The Islamic State is also known as ISIL and ISIS.

A statement by Lt. Gen. James L. Terry, the coalition commander, said that “this will further enable Iraqi forces under Iraqi command to maneuver and defeat ISIL in the vicinity of Tikrit.”

Lt. Col. Brian Fickel, a spokesman for Gen. Lloyd Austin III, head of the U.S. Central Command, said Iraqi security forces were in command of the Tikrit operation and that the United States and its allies were coordinating with those forces, not ­Iranian-backed paramilitaries.

“We do not coordinate our operations in any way with Iran,” he said.

A U.S. defense official said that American aircraft were involved in the Tikrit operation with at least two other countries that are part of the coalition operating inside Iraq. The coalition includes European allies and Canada and Australia.

The official, who was not authorized to publicly discuss the subject, said that initial targets were pre-set, rather than selected in response to on-the-fly communications with Iraqi security ­forces, but that the latter process would come as the campaign continued. Fewer than a dozen targets were likely to be hit in overnight sorties Wednesday and Thursday, the official said.

Abadi announced the new push for Tikrit in a televised address Wednesday night, saying that the city’s “hour of salvation” had come. He did not specifically mention coalition airstrikes, but he said, “We will liberate each inch of Iraq. The victory of Iraq is being achieved by Iraqis, hero Iraqis . . . with support from friendly countries and the international coalition.”

The decision to start airstrikes could be met with resistance from Shiite forces on the ground. Before the U.S. announcement was made, militia leaders — some of whom fought U.S. troops during the Iraq war — had already objected.

Iraq should not request air assistance around Tikrit because Iraqi army troops “don’t have any involvement” in the operation, said Hadi al-Amiri, leader of the Badr Organization, one of the Shiite groups that joined the fight along with Iraqi Shiite volunteers known as the Popular Mobilization Units.

“With all my respect to the army, the one who is doing the operation are the federal police and popular mobilization,” Amiri said in an interview in the city of Samarra, less than 40 miles from Tikrit. “They are now the real force.”

He complained that the presence of U.S. planes would force surveillance drones belonging to the militias out of the air, since there is no direct coordination, apart from through the Iraqi government, between the coalition and the Iranian-backed groups.

Brig. Gen. Abed al-Maliki, an Iraqi army commander in Samarra, conceded the military’s minor role in Tikrit. “The operation was started by the popular mobilizations and has [been] continued” by them, he said. “They have better weapons and ammunition than us; they have more support.”

After making an initial rapid advance across territory surrounding militant-held Tikrit that led to the issuance of triumphant reports of victory, the Iraqi offensive has been stalled for more than a week amid high casualties. If Iraqi forces cannot operate efficiently in capturing Tikrit, the all-important battle for Mosul is likely to be postponed.

In recent congressional testimony, when it appeared the Iraqis were on the verge of taking Tikrit, Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, said U.S. officials were watching carefully to see how the Shiite militia forces would treat Sunni residents returning to the city. In the Sunni-dominated western part of Iraq, tribal leaders have been hesitant to turn against the Islamic State because of fear the militants would be replaced by Shiites in both the militias and the Iraqi military.

Human rights groups in recent days have documented the Shiite pursuit of a scorched earth policy in areas already liberated from the Islamic State. After U.S. airstrikes drove the militants out of the town of Amerli, in northeastern Iraq, late last summer, the militias went on a sectarian rampage, burning and bulldozing thousands of homes and other buildings in dozens of Sunni villages.

DeYoung and Ryan reported from Washington. Mustafa Salim in Baghdad contributed to this report.

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