President Obama, saying that he was acting to avert a threatened genocide, announced that he had authorized airstrikes against Sunni militants who have advanced quickly across northern Iraq, driving tens of thousands of people out of their towns and villages.
In addition to the threat of airstrikes, Obama said U.S. cargo planes had dropped supplies to the displaced Iraqis in the area of Mt. Sinjar, who faced the risk of imminent starvation.
"Today, America is coming to help," Obama said in brief remarks from the White House.
Obama repeated his vow that no American combat troops would be sent back to Iraq, saying that "as commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq."
But Obama shed little light on how long the U.S. is prepared to conduct airstrikes and what he would do if air power alone fails to halt the militants' advance.
The displaced Iraqis, primarily members of the Yazidi religious sect, "have fled for their lives," Obama said. "And thousands -- perhaps tens of thousands -- are now hiding high up on the mountain, with little but the clothes on their backs. They're without food; they're without water. People are starving. And children are dying of thirst."
Meanwhile, the militants "have called for the systematic destruction of the entire Yazidi people, which would constitute genocide."
"When we face a situation like we do on that mountain, with innocent people facing the prospect of violence on a horrific scale, when we have a mandate to help -- in this case, a request from the Iraqi government -- and when we have the unique capabilities to help avert a massacre, then I believe the United States of America cannot turn a blind eye," he said.
No U.S. airstrikes have yet taken place, White House and Pentagon officials said. Obama has authorized the military to attack forces of the Islamic State if that becomes necessary to break the siege of the Mt. Sinjar area, or if needed to protect U.S. personnel in Irbil, the capital of the semiautonomous Kurdish region in northern Iraq, officials said.
"There is no decision that I take more seriously than the use of military force," Obama said. "I have been careful to resist calls to turn time and again to our military. But when the lives of American citizens are at risk, we will take action."
The strikes are geographically restricted to Iraq, a senior White House official said, and won't cross over into Syria or any other country where the Islamic State fighters are operating. Strikes in Iraq would be justified under international law because the Iraqi government has requested the assistance, the official said.
In a statement issued shortly before Obama spoke, Pentagon officials said U.S. planes had dropped "critical meals and water for thousands of Iraq citizens."
Three cargo planes -- a C-17 and two C-130s -- dropped a total of 72 bundles of supplies, including 5,300 gallons of fresh water and 8,000 rations, Pentagon officials said. The cargo planes were escorted by two F/A-18 fighter jets.
The aircraft were over the drop area for less than 15 minutes at a low altitude, the officials said. All the planes "safely exited the immediate airspace over the drop area," a Pentagon statement said.
In recent days, tens of thousands of the Yazidi sect fled towns in the north that were taken over by fighters from the Islamic State.
The Yazidis, as well as many Christians, moved north into Kurdish-held areas of Iraq to escape the militants. But about 40,000 remain trapped on Mt. Sinjar and are in extremely dire circumstances, according to Kurdish officials and international relief groups.
The U.S. airdrops are designed to deliver emergency supplies to displaced Iraqis in an effort to stave off what White House Press Secretary Josh Earnest described as a potential "humanitarian catastrophe."
White House officials notified congressional leaders late Thursday of the airdrops and the threatened airstrikes.
The moves drew statements of approval from Democrats and mixed responses from Republicans, some of whom said Obama was doing too little, too late.
Democratic Sen. Carl Levin of Michigan, chairman of the Senate Armed Services Committee, said Obama's justification for military action was "surely sufficient."
"It is helpful that the government of Iraq has requested our assistance, and it would also be helpful under the circumstances, though not necessary, for a number of neighboring countries to publicly support our actions," Levin said.
Sen. Marco Rubio (R-Fla.) called the president's actions "the right decision," but said the administration should follow up by increasing military assistance to the Kurds. The administration has been hesitant about further arming the Kurds, who have threatened to secede from Iraq, a move that U.S. officials fear would further destabilize a troubled region.
Others in the GOP remained skeptical that the administration was taking strong enough action.
Sens. Lindsey Graham of South Carolina and John McCain of Arizona, leaders of the hawkish wing of the GOP, said in a statement that they supported the effort, but that the actions "are far from sufficient to meet the growing threat" posed by the Islamic State.
"The president is right to provide humanitarian relief to the Iraqi civilians stranded on Mount Sinjar and to authorize military strikes," they said. But "we need a strategic approach, not just a humanitarian one."
"A policy of containment will not work" against the militants, they said, calling for airstrikes not just in Iraq, but also in Syria and for supplying more weapons to the Kurds and Syrian forces who are fighting the militants. "The longer we wait to act, the worse this threat will become, as recent events clearly show.