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 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 afterattacks 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.” As Mr. Obama considered new strikes, the White House began its diplomatic campaign to enlist allies and neighbors in the region to increase their support for Syria’s moderate opposition and, in some cases, to provide support for possible American military operations. The countries likely to be enlisted include Australia, Britain, Jordan, Qatar, Saudi Arabia, Turkey and the United Arab Emirates, officials said. The officials, who asked not to be named discussing sensitive internal deliberations, said they expected that Britain and Australia would be willing to join the United States in an air campaign. The officials said they also wanted help from Turkey, which has military bases that could be used to support an effort in Syria. Turkey is a transit route for foreign fighters, including those from the United States and Europe who have traveled to Syria to join ISIS. Administration officials said they are now asking officials in Ankara to help tighten the border. The administration is also seeking intelligence and surveillance help from Jordan as well as financial help from Saudi Arabia, which bankrolls groups in Syria that are fighting President Bashar al-Assad. On Monday the Pentagon began surveillance flights over Syria in an effort to collect information on possible ISIS targets as a precursor to airstrikes, a senior official said. The Syrian Observatory for Human Rights, an organization that monitors the humanitarian consequences of the conflict in Syria, reported that “non-Syrian spy planes” on Monday carried out surveillance of ISIS positions in the eastern province of Deir Ezzor. Although America’s allies in the region have plenty of reasons to support an intensified effort against ISIS, analysts said, the United States will have to navigate tensions among them. “One of the problems is that different countries have different clients among the fighting groups in Syria,” said Robert S. Ford, a former American ambassador to Syria. “To get them all to work together, the best thing would be for them to pick one client and funnel all the funds through that client. You’ve got to pick one command structure.” But persuading counties to help the United States in a military campaign in Syria will require more effort, administration officials said. Turkey, for example, is in the midst of a political transition, with Prime Minister Recep Tayyip Erdogan ascending to the presidency. His likely successor as prime minister, Ahmet Davutoglu, has been deeply immersed in Syria as foreign minister. The White House, meanwhile, has been unable to win Senate confirmation of a new ambassador to Turkey, John Bass, leaving the post vacant at a critical time. Saudi Arabia and the Persian Gulf emirates are important as a source of funding for the rebels, but there are strains among them. Qatar, for example, helped negotiate the release of an American hostage, Peter Theo Curtis, who was being held by a less extreme militant group, the Nusra Front. But Saudi Arabia does not talk to the Nusra Front, and the Obama administration has sought to navigate between the feuding gulf countries. Enlisting the Sunni neighbors of Syria is crucial, experts said, because airstrikes alone will not be enough to push back ISIS. The administration, Mr. Ford said, needs to pursue a sequential strategy that begins with gathering intelligence, followed by targeted airstrikes, more robust and better coordinated support for the moderate rebels, and finally, a political reconciliation process similar to that underway in Iraq. The White House is also debating how to satisfy a second constituency, Congress. Mr. Obama’s advisers are considering whether to seek congressional authorization for expanded military action and if so, under what legal rationale. Lawmakers had been reluctant to vote on airstrikes in Iraq, but several have begun arguing that the broader action being contemplated by Mr. Obama would demand a vote in Congress. “I do not believe that our expanded military operations against ISIL are covered under existing authorizations from Congress,” said Senator Tim Kaine of Virginia, a Democratic member of the Senate Foreign Relations Committee. Senator Bob Corker, the Tennessee Republican who is the ranking member of the Foreign Relations Committee, said on MSNBC that Congress needed to “own” any further military action against the militants. On Tuesday, Defense Secretary Chuck Hagel said in a statement that seven Western countries had pledged to provide weapons and ammunition to Kurdish forces who are fighting ISIS in northern Iraq. Albania, Canada, Croatia, Denmark, France, Italy and Britain have committed to sending arms and equipment to the Kurds, Mr. Hagel said, adding that operations would “accelerate in coming days with more nations also expected to contribute.” Rear Adm. John Kirby, the Pentagon press secretary, said Albania and Britain had started moving supplies to the Kurds.