WASHINGTON — The United States has begun to mobilize a broad coalition of allies behind potential American military action in Syria and is moving toward expanded airstrikes in northern Iraq, administration officials said on Tuesday.

President Obama, the officials said, was broadening his campaign against the Sunni militants of the Islamic State in Iraq and Syria and nearing a decision to authorize airstrikes and airdrops of food and water around the northern Iraqi town of Amerli, home to members of Iraq’s Turkmen minority. The town of 12,000 has been under siege for more than two months by the militants.

“Rooting out a cancer like ISIL won’t be easy, and it won’t be quick,” Mr. Obama said in a speech on Tuesday to the American Legion in Charlotte, N.C., using an alternative name for ISIS. He said that the United States was building a coalition to “take the fight to these barbaric terrorists,” and that the militants would be “no match” for a united international community.

Administration officials characterized the dangers facing the Turkmen, who are Shiite Muslims considered infidels by ISIS, as similar to the threat faced by thousands of Yazidis, who were driven to Mount Sinjar in Iraq after attacks by the militants. The United Nations special representative for Iraq, Nickolay Mladenov, said in a statement three days ago that the situation in Amerli “demands immediate action to prevent the possible massacre of its citizens.”