WASHINGTON — President Obama on Thursday announced he had authorized limited airstrikes against Islamic militants in Iraq, scrambling to avert the fall of the Kurdish capital, Erbil, and returning the United States to a significant battlefield role in Iraq for the first time since the last American soldier left the country at the end of 2011.

Speaking at the White House on Thursday night, Mr. Obama also said that American military aircraft had dropped food and water to tens of thousands of Iraqis trapped on a barren mountain range in northwestern Iraq, having fled the militants, from the Islamic State of Iraq and Syria, who threaten them with what Mr. Obama called “genocide.”

“Earlier this week, one Iraqi cried that there is no one coming to help,” Mr. Obama said in a somber statement delivered from the State Dining Room. “Well, today America is coming to help.”

The president insisted that these military operations did not amount to a full-scale re-engagement in Iraq. But the relentless advance of the militants, whom he described as “barbaric,” has put them within a 30-minute drive of Erbil, raising an immediate danger for the American diplomats, military advisers and other citizens who are based there.

“As commander in chief, I will not allow the United States to be dragged into another war in Iraq,” said Mr. Obama, who built his run for the White House in part around his opposition to the war in Iraq.

While Mr. Obama has authorized airstrikes, American officials said there had not yet been any as of late Thursday. In addition to protecting Americans in Erbil and Baghdad, the president said he had authorized airstrikes, if necessary, to break the siege on Mount Sinjar, where tens of thousands of Yazidis, a religious minority group closely allied with the Kurds, have sought refuge.

The aircraft assigned to dropping food and water over the mountainside were a single C-17 and two C-130 aircraft. They were escorted by a pair of F-18 jet fighters, the administration official said. The planes were over the drop zone for about 15 minutes, and flew at a relatively low altitude. They flew over the Mount Sinjar area for less than 15 minutes, Pentagon officials said, and dropped a total of 5,300 gallons of fresh drinking water and 8,000 meals ready to eat. Mr. Obama, officials said, delayed announcing the steps he intended to take in Iraq until the planes had safely cleared the area.

A senior administration official said that the humanitarian effort would continue as needed, and that he expected further airdrops. “We expect that need to continue,” he said.