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. 
 
 The official said that as conditions in Iraq deteriorated in recent days, the United States had worked with Iraqi security forces and Kurdish fighters to coordinate the response to militant advances. The official said the cooperation had included airstrikes by Iraqi forces against militant targets in the north. 
 
 Kurdish and Iraqi officials said that airstrikes were carried out Thursday night on two towns in northern Iraq seized by ISIS — Gwer and Mahmour, near Erbil. Earlier on Thursday, The New York Times quoted Kurdish and Iraqi officials as saying that the strikes were carried out by American planes. 
 
 While the militants are not believed to have surface-to-air missiles, they do have machine guns that could hit planes flying at a low altitude, said James M. Dubik, a retired Army lieutenant general who oversaw the training of the Iraqi Army in 2007 and 2008. 
 
 “These are low and slow aircraft,” General Dubik said. At a minimum, he said, the United States must be prepared for “some defensive use of air power to prevent” the militants from attacking American planes, or going after the humanitarian supplies. 
 
 Advertisement Continue reading the main story 
 
 For Mr. Obama, who has steadfastly avoided being drawn into the sectarian furies of the Middle East, the decision raises a host of difficult questions, injecting the American military into Iraq’s broader political struggle — something Mr. Obama said he would not agree to unless Iraq’s three main ethnic groups agreed on a national unity government. 
 
 The decision could also open Mr. Obama to charges that he is willing to use American military might to protect Iraqi Christians and other religious minorities but not to prevent the slaughter of Muslims by other Muslims, either in Iraq or neighboring Syria. 
 
 But the president said the imminent threat to Erbil and the dire situation unfolding on Mount Sinjar met both his criteria for deploying American force: protecting American lives and assets, and averting a humanitarian disaster. 
 
 Video 
 
 “When we have the unique capacity to avert a massacre, the United States cannot turn a blind eye,” he said. 
 
 Mr. Obama has been reluctant to order direct military action in Iraq while Prime Minister Nuri Kamal al-Maliki remains in office, but in recent weeks there have been repeated pleas from the Kurdish officials for weapons and assistance as ISIS militants have swept across northwestern Iraq. The militants, an offshoot of Al Qaeda, view Iraq’s majority Shiite and minority Christians and Yazidis as infidels. 
 
 Deliberations at the White House went on all day Thursday as reports surfaced that administration officials were considering either humanitarian flights, airstrikes or both. 
 
 Shortly after 6 p.m., the White House posted a photo of Mr. Obama consulting his national security team in the Situation Room. To his right was the chairman of the Joint Chiefs of Staff, Gen. Martin E. Dempsey. Watching from across the table were Susan E. Rice, the national security adviser, and her principal deputy, Antony J. Blinken. On the wall behind them, the clock recorded the time: 10:37 a.m. 
 
 Mr. Obama made only one public appearance, a rushed visit to Fort Belvoir, Va., where he signed into law a bill expanding access to health care for veterans. But aides suggested he might make a statement Thursday night. Before getting into his limousine, Mr. Obama was observed holding an intense conversation with his chief of staff, Denis R. McDonough, stabbing his finger several times for emphasis. 
 
 Later, Mr. McDonough telephoned the House speaker, John A. Boehner, to inform him of the president’s plans, and other White House officials spoke with lawmakers — all in an effort to avoid bruised feelings like those that followed the prisoner swap for Sgt. Bowe Bergdahl. 
 
 Advertisement Continue reading the main story 
 
 Administration officials said on Thursday that the crisis on Mount Sinjar in northwestern Iraq had forced their hand. Some 40 children have already died from the heat and dehydration, according to Unicef, while as many as 40,000 people have been sheltering in the bare mountains without food, water or access to supplies. 
 
 Still, offensive strikes on militant targets around Erbil and Baghdad would take American involvement in the conflict to a new level — in effect, turning the American Air Force into the Iraqi Air Force. 
 
 “The White House is going to recognize that the need to commit air power to Iraq, even for a purely humanitarian mission, is going to open them up to greater criticism for their disengagement from Iraq,” said Kenneth M. Pollack, a senior fellow at the Brookings Institution. “So they will do their damnedest not to get further involved in Iraq because that would just further validate those criticisms.” 
 
 Ever since Sunni militants with ISIS took over Mosul, Iraq’s second-largest city, on June 10, Iraqis have feared that Baghdad, to the south, was the insurgents’ ultimate goal. But in recent weeks, the militant group has concentrated on trying to push the Kurds back from areas where Sunnis also live along the border between Kurdistan and Nineveh Province. 
 
 It has taken on the powerful Kurdish militias, which were thought to be a bulwark against the advance, and which control huge oil reserves in Kurdistan and broader parts of northern Iraq. An administration official said the United States would expedite the delivery of weapons to the Kurds. 
 
 For Mr. Obama, the suffering of the refugees on the mountainside appeared to be a tipping point. He spoke in harrowing terms about their dire circumstances, saying thousands of people were “hiding high up on the mountain, with little but the clothes on their backs.” 
 
 “They’re without food, they’re without water,” he said. “People are starving. And children are dying of thirst. These innocent families are faced with a horrible choice: descend the mountain and be slaughtered, or stay and slowly die of thirst and hunger.” ||||| In a statement made Thursday night, President Obama said he has authorized military strikes on Islamic State militants if they continue their advancement on the city of Irbil, where military personnel and the U.S. consulate are located. (AP) 
 
 President Obama said Thursday night that he has authorized two new, major actions by the U.S. military in Iraq: airstrikes against militants with the Islamic State if U.S. interests or personnel are threatened, and humanitarian assistance for thousands of civilians who have fled the Islamist advance in northwest Iraq and are now trapped on a mountain. 
 
 Even those two specific missions are more complicated than that, however. 
 
 First, the U.S. humanitarian mission on Mount Sinjar began Thursday with airdrops by military aircraft, but it could require targeted airstrikes in order to “break the siege” at the base of the mountain and allow authorities to provide more assistance to the civilians, Obama said. They would be conducted in coordination with the Iraqi government and Iraqi Kurdish security forces known as pesh merga. 
 
 Second, the advance of the Islamic State near Irbil appears to be a line in the sand for the Obama administration. The United States has both diplomats and military advisers in the city, which is the capital of the Kurdistan region of Iraq. The United States also has a consulate in Irbil, creating enough interests that the United States wants to protect the city from harm. 
 
 “To stop the advance on Irbil, I’ve directed our military to take targeted strikes against [Islamic State] terrorist convoys should they move toward the city,” Obama said. “We intend to stay vigilant, and take action if these terrorist forces threaten our personnel or facilities anywhere in Iraq, including our consulate in Irbil and our embassy in Baghdad.” 
 
 
 
 In this image released by The White House, President Obama meets with the National Security Council in the Situation Room of the White House on Thursday in Washington. (AP Photo/The White House, Pete Souza) 
 
 Here’s what we know now about the specifics of these missions: 
 
 Humanitarian air drops 
 
 The U.S. military launched one C-17 and two C-130 cargo planes near Mount Sinjar on Thursday, dropping 8,000 prepackaged meals and 5,300 gallons of water, senior U.S. officials said. The aircraft were accompanied by two F/A-18 fighter jets, which came out of an undisclosed air base in the Middle East. The Pentagon did not indicate which units or branch of service operated any of the aircraft. 
 
 Pentagon officials said in a statement that the three cargo planes released a total of 72 bundles of supplies. They flew over the drop area for less than 15 minutes, and left the area before Obama announced the actions in his 9:30 p.m. address. 
 
 Senior U.S. officials left open the possibility that the United States could conduct more airdrops in coming days if it’s needed. That appears possible, considering human rights workers have estimated that up to 40,000 civilians may be trapped on the mountain, unable to return their homes for fear of being killed by militants. 
 
 “We feel that this is a unique and urgent humanitarian challenge,” one senior officials said Thursday night. 
 
 
 
 This image made from video taken on Sunday shows Iraqis people from the Yazidi community arriving in Irbil in northern Iraq after Islamic militants attacked the towns of Sinjar and Zunmar. Around 40,000 people crossed the bridge of Shela in Fishkhabur into the northern Kurdish region of Iraq after being given an ultimatum by Islamic militants to either convert to Islam, pay a security tax, leave their homes, or die. (AP Photo via AP video) 
 
 Preparing for airstrikes 
 
 The advance of militants across northern Iraq has been watched closely this week, but a new wave of attacks threatened Irbil in a way it had not been before, U.S. officials said. That led the president to authorize airstrikes if necessary. 
 
 U.S. officials also said that with the increased threat now facing Irbil, the United States has either surveillance drones or manned aircraft flying over the area almost constantly, keeping tabs on militant movements. 
 
 Defense Secretary Chuck Hagel said in a statement Thursday night that U.S. military advisers in Iraq will continue to assess ways to help train and assist Iraqi forces, and will provide undisclosed “increased support” once Iraq has formed a new government. Most of the troops there are U.S. Army Special Forces. 
 
 Outgoing Iraqi Prime Minister Nouri al-Malikihas faced fierce criticism for appointing friends to key positions, allowing corruption and cutting Iraq’s Sunni population out of the political process, fueling the insurgency. A new prime minister could be selected in coming days, U.S. officials signaled Thursday night. 
 
 UPDATED, Friday, Aug. 8, 9 a.m.: U.S. airstrikes in Iraq have begun after militants launched artillery rounds at Kurdish troops defending Irbil. Rear Adm. John Kirby, the Pentagon press secretary, posted this tweet: ||||| WASHINGTON (AP) — U.S. fighters dropped bombs on Islamic militants in Iraq Friday, the Pentagon said, carrying out President Barack Obama's promise of military force to counter the advancing militants and confront the threat they pose to Iraqi civilians and Americans still stationed there. 
 
 FILE - This June 20, 2007, file photo shows a C-130-J transport aircraft over Little Rock Air Force Base in Jacksonville, Ark. President Barack Obama authorized U.S. airstrikes in northern Iraq, Thursday,... (Associated Press) 
 
 President Barack Obama approaches the podium to speak about the situation in Iraq in the State Dining Room at the White House in Washington, Thursday, Aug. 7, 2014. Obama says he has authorized the U.S.... (Associated Press) 
 
 President Barack Obama speaks about the situation in Iraq in the State Dining Room at the White House in Washington, Thursday, Aug. 7, 2014. Obama says he has authorized the U.S. military to launch targeted... (Associated Press) 
 
 President Barack Obama speaks about the situation in Iraq in the State Dining Room at the White House in Washington, Thursday, Aug. 7, 2014. Obama says he's authorized the U.S. military to carry out airstrikes... (Associated Press) 
 
 President Barack Obama speaks about the situation in Iraq in the State Dining Room at the White House in Washington, Thursday, Aug. 7, 2014. (AP Photo/Charles Dharapak) (Associated Press) 
 
 Pentagon press secretary Rear Adm. John Kirby said that two F/A-18 jets dropped 500-pound bombs on a piece of artillery and the truck towing it. Kirby said the fighters had taken off from the aircraft carrier USS George HW Bush in the Persian Gulf to conduct the mission. He said it wasn't clear how many militants might have been killed in the strike. 
 
 The Pentagon said the militants were using the artillery to shell Kurdish forces defending Irbil. 
 
 For the United States, it was a re-engagement in the long sectarian war from which American combat forces had been withdrawn — on President Barack Obama's orders — in late 2011. 
 
 In a televised speech Thursday night, Obama threatened to renew U.S. military involvement. At the same time, he announced that U.S. military planes already had carried out airdrops of food and water, at the request of the Iraqi government, to tens of thousands of Iraqi religious minorities atop a mountain surrounded by militants and desperately in need of supplies. 
 
 "America is coming to help," Obama declared. 
 
 Speaking to reporters while traveling in India Friday, Defense Secretary Chuck Hagel said the U.S. military has sufficient intelligence resources and assets in place to launch strikes by both manned and unmanned aircraft in the region. 
 
 Asked if the Islamic State group could successfully hide among civilians to evade strikes, Hagel said if the Islamic State moves against Irbil, Baghdad or the refugees trapped on a mountain, "it's pretty clear who they are, and they would be pretty identifiable where our airstrikes could be effective." 
 
 The Yazidis, who follow an ancient religion with ties to Zoroastrianism, fled their homes after the Islamic State group issued an ultimatum to convert to Islam, pay a religious fine, flee their homes or face death. 
 
 "Earlier this week, one Iraqi in the area cried to the world, 'There is no one coming to help.' Well, today, America is coming to help," Obama said. "We're also consulting with other countries — and the United Nations — who have called for action to address this humanitarian crisis." 
 
 The announcement reflected the deepest American engagement in Iraq since U.S. troops left. 
 
 Obama, who made his remarks in a steady and somber tone, has staked much of his legacy as president on ending what he once called the "dumb war" in Iraq. 
 
 Mindful of the public's aversion to another lengthy war, Obama acknowledged that the prospect of a new round of U.S. military action would be a cause for concern among many Americans. He vowed anew not to put American combat troops back on the ground in Iraq and said there was no U.S. military solution to the crisis. 
 
 "As commander in chief, I will not allow the United States to be dragged into fighting another war in Iraq," Obama said. 
 
 Even so, he outlined a rationale for airstrikes in the event the Islamic State militants advanced on American troops in Irbil and the U.S. consulate there in the Kurdish region of Iraq. The troops were sent to Iraq earlier this year as part of the White House response to the extremist group's swift movement across the border with Syria and into Iraq. 
 
 "When the lives of American citizens are at risk, we will take action," Obama said. "That's my responsibility as commander in chief." 
 
 He said he had also authorized the use of targeted military strikes if necessary to help the Iraqi security forces protect civilians. 
 
 The Pentagon said the airdrops were performed by one C-17 and two C-130 cargo aircraft that together delivered a total of 72 bundles of food and water. They were escorted by two F/A-18 fighters from an undisclosed air base in the region. 
 
 The planes delivered 5,300 gallons of fresh drinking water and 8,000 pre-packaged meals and were over the drop area for less than 15 minutes at a low altitude. 
 
 The president cast the mission to assist the Yazidis as part of the American mandate to assist around the world when the U.S. has the unique capabilities to help avert a massacre. 
 
 In those cases, Obama said, "we can act carefully and responsibly to prevent a potential act of genocide." 
 
 Officials said the U.S. was prepared to undertake additional humanitarian airdrops if necessary, though they did not say how quickly those missions could occur. 
 
 Administration officials had said earlier they believed unilateral U.S. strikes would be consistent with international law, in part because the Iraqi government has asked for Washington to take military action. They also said Obama had the constitutional authority to act on his own in order to protect American citizens. 
 
 Obama had similarly authorized strikes in Syria last summer after chemical weapons were deployed, but those attacks were never carried out, in part because of domestic political concerns and also because an international agreement to strip Syria of its stockpiles of the deadly gases. 
 
 The president has also faced persistent calls to take military action in Syria on humanitarian grounds, given that more than 170,000 people have been killed there. 
 
 Critics, including some Republicans in Congress, have argued that Obama's cautious approach to Syria has allowed the Islamic State group to flourish there, growing strong enough to move across the border with Iraq and make swift gains. 
 
 Republican Sens. John McCain of Arizona and Lindsey Graham of South Carolina praised Obama's proposed actions Thursday night but said much more will be necessary. 
 
 "This should include the provision of military and other assistance to our Kurdish, Iraqi and Syrian partners" who are fighting the militants, airstrikes against the militants' leaders and forces and support for Sunni Iraqis who seek to resist the extremists, they said in a statement. 
 
 In light of the militants' advances, Obama dispatched about 800 U.S. forces to Iraq earlier this year, with those troops largely split between joint operation centers in Baghdad and Irbil. 
 
 More than half are providing security for the embassy and U.S. personnel. American service members also are involved in improving U.S. intelligence, providing security cooperation and conducting assessments of Iraqi capabilities. 
 
 ___ 
 
 Associated Press writers Josh Lederman and Darlene Superville contributed to this report. 
 
 ___ 
 
 Follow Julie Pace at http://twitter.com/jpaceDC and Robert Burns at http://twitter.com/RobertBurnsAP
– The US launched airstrikes against Iraq's Islamic State militants this morning. "US military aircraft conduct strike on ISIL artillery," tweeted Rear Adm. John Kirby. "Artillery was used against Kurdish forces defending (Irbil), near US personnel." He confirms that F/A-18 jets dropped a pair of laser-guided 500-pound bombs on militants towing artillery at about 6:45am Eastern, reports the AP. US military trainers are stationed nearby. More: The F/A-18s were based on the aircraft carrier USS George HW Bush in the Persian Gulf and took off on the orders of the commander of US Central Command, CNN reports. (President Obama authorized airstrikes last night.) It's unclear how many militants were killed in the strikes. John Kerry, speaking today in Afghanistan, echoed Obama's language, saying the Islamic State's "campaign of terror against the innocent, including the Yazidi and Christian minorities, and its grotesque targeted acts of violence show all the warning signs of genocide." Meanwhile, the AP reports that Iraqi and Kurdish officials are welcoming last night's humanitarian aid drops for Iraqis trapped by the militants in the mountains. "We thank Barack Obama," said one Kurdish official in northern Iraq, while a rep for the Iraqi Ministry of Migration and Displacement said the drops were "just in time."