Kalak, Iraq — The United States returned Friday to war in Iraq, bombing Islamic State fighters encroaching on the capital of Iraqi Kurdistan, where scores of American diplomats and military advisers are based, and making a second drop of food and water to refugees stranded in mountains in northern Iraq.

The Pentagon announced the second humanitarian airdrop late Friday and said it contained 28,224 military meals-ready-to-eat and 1,522 gallons of water for the refugees, who are believed to number in the tens of thousands.

The strikes by U.S. fighter jets and drones, following a U.S. humanitarian air drop of food and water that was announced Thursday night in Washington, were the first known American military operations in Iraq since U.S. forces withdrew in December 2011 and underscored the difficulty that President Barack Obama has encountered in trying to limit U.S. involvement in the sectarian upheavals roiling the Middle East.

The airstrikes targeted an Islamic State artillery position, a mortar site and what the Pentagon called a “stationary convoy.” Casualties among the Islamic State fighters were uncertain; one of the sites was hit twice when a U.S. drone aircraft detected fighters returning to the area.

The attacks seemed to bolster the morale among Kurdish fighters, who on Thursday had been downcast about the prospect of an imminent Islamic State attack. But on Friday, the Kurdish troops, who’ve established a defensive line outside Kalak, a town 25 miles northwest of Irbil, talked excitedly about the day’s events. They said they could hear the explosions from the strikes and speculated in which of three nearby Islamic State-held towns the explosions had occurred.

Still, the streets of Kalak were empty as residents stayed indoors to avoid Islamic State shellfire, which reportedly continued regularly throughout the day despite the U.S. strikes.

Irbil remained tense, in spite of Obama’s vow that United States air power would block any Islamic State advance on the capital, where the U.S. maintains a large consulate, a recently expanded CIA station and a joint operations center where 40 U.S. military advisers were recently assigned.

International oil companies, which maintain a huge presence in Irbil, put their employees on the highest alert as they prepare evacuation plans, and Exxon and Chevron, which had confined most of their employees to their homes under heavy security, brought planes to Irbil’s international airport for evacuation flights should conditions deteriorate, according to industry sources.

The British Embassy warned its citizens to leave Irbil and Dohuk, another city close to the fighting.

Kurdish security officials, speaking on the condition of anonymity because they weren’t authorized to speak publicly, said they’d begun arresting Arab Sunni Muslims suspected of Islamic State sympathies, and Kurdish security forces in civilian clothes and unmarked cars could be seen conducting searches and detentions.

“We have real concerns about Daash posing as refugees,” said one security official, using the derogatory Arabic acronym for the Islamic State. “We have been watching closely for them and caught many trying to enter our territory.”

In Washington, the Federal Aviation Authority announced that it was banning U.S. commercial aircraft from overflying Iraq because of the violence. Some European and Arab airlines have suspended flights to and from Irbil.

Meanwhile, U.S. and humanitarian aid groups began to assess the effectiveness of the first U.S. air drop of water and food to the thousands of members of Iraq’s Yazidi religious minority who are trapped in the mountains, where they fled to escape the Islamic State capture of the nearby city of Sinjar.

Escorted by two F/A-18s, three low-flying U.S. cargo aircraft had dropped 5,300 gallons of drinking water and enough military meals-ready-to-eat for about 8,000 people early Friday, U.S. officials said.

The second mission, dropped on the mountain early today, also involved two F/A-18 fighter jets, a C-17 and two C-130s, but included far more food and far less water. There was no explanation offered in the Pentagon statement for the revised mix.

As is typical of such a difficult logistical challenge, not all of the 72 pallets dropped in the first mission made it to the refugees, a senior military official told McClatchy, and the 63 pallets of food and water that did were likely to be quickly exhausted by the refugees, whose precise number is unknown. The official spoke to McClatchy only under the condition of anonymity because of the sensitivity of the issue. At least some of the missing pallets likely ended up in Islamic State hands, the official said.

A senior administration official, briefing reporters under the condition of anonymity, told reporters late Thursday that the United States is skeptical of a United Nations estimate that 40,000 people were on the mountain. But Joe Stork, deputy director of the Middle East and North Africa Division of Human Rights Watch, said that there might be more than 100,000.

“They fled when the peshmerga withdrew from their posts early Sunday morning,” Stork told McClatchy in a telephone interview from Irbil. “The 100,000-plus in the mountains have no water, food or shelter from temperatures as high as 120 degrees Fahrenheit.”

Senior administration officials, who spoke anonymously because of White House ground rules, said that Obama authorized airstrikes if necessary to help peshmerga and Iraqi security forces break the Islamic State’s encirclement of the mountains. But there was no word that such an effort was under way, and video shot by a Kurdish television station showed crowds of refugees clamoring for assistance.

U.S. officials continued Friday to denounce the Islamic State’s encirclement of the Yazidis. Secretary of State John Kerry, in Afghanistan, used the word “genocide” to condemn the Islamic State’s “campaign of terror against the innocent, including the Yazidi and Christian minorities, and its grotesque and targeted acts of violence” which, he said, “bear all the warning signs and hallmarks of genocide.”

“For anyone who needed a wake-up call, this is it,” he said.

The sudden onset of new U.S. military action in Iraq raised concerns among some that the United States would be drawn into a deeper involvement. But there was also recognition that not responding to the Islamic State might have dire consequences.

“It’s a slippery slope, but given the stakes, it’s hard to imagine how the president could avoid this course of action without accepting a moderate to high chance of total regime collapse,” said Stephen Long, a national security professor at the University of Richmond in Virginia.

White House spokesman Josh Earnest, however, insisted that the mission would not be long. He repeated Obama’s pledge that no U.S. combat troops would be deployed.

“The authorization that the president ... has given for military action is very limited in scope,” said Earnest. He declined, however, to give a specific timetable, explaining that Obama has not set an end date.

In Baghdad, meanwhile, Obama’s call on Iraq’s leaders to end squabbles that have hampered the formation of a new government following April polls seemed to fall on deaf ears, with Prime Minister Nouri al-Maliki showing no sign of heeding calls not to seek another term.

Al-Maliki, who issued no comment on the airstrikes, is widely blamed for the current crisis for failing to act early against the Islamic State and for pursuing policies that favored his majority Shiite Muslim sect and disenfranchised minority Sunnis.

The airstrikes triggered a torrent of threats against Americans on social media, some of them using grotesque photographs of severed heads and mangled bodies of dead U.S. troops. Many were posed on Twitter using the hashtag #AmessagefromISIStoUS. It wasn’t clear if any were authentic warnings from the Islamic State.

“This is a message for every American citizen. You are the target of every Muslim in the world wherever you are,” said a tweet signed by “alsarim1.” “We swear that you won’t be safe in your countries whilst your aircraft kill our people in Iraq,” said another, posted by sunnanews.

The Pentagon said Friday’s airstrikes began about 1:45 p.m. local time when two F/A-18 fighter jets flying off the aircraft carrier George H.W. Bush dropped 500-pound laser-guided bombs on a “mobile artillery piece” that was being used to fire on Kurdish peshmerga fighters who were defending Irbil.

The second U.S. airstrike took placed about 5 p.m. local time when a U.S. drone fired a Hellfire missile at an Islamic State mortar position, according to Pentagon spokesman Navy Rear Adm. John Kirby. When extremist fighters returned moments later, a second drone launched another missile that “successfully eliminated” them, Kirby said.

About an hour and 20 minutes later, four F/A-18s struck an Islamic State mortar position and a convoy of seven vehicles, Kirby said.

“The planes executed two planned passes. On both runs, each aircraft dropped one laser-guided bomb, making a total of eight bombs dropped on target, neutralizing the mortar and the convoy,” he said.

Obama’s decision to intervene came after the Islamic State on Thursday captured the Mosul Dam, the largest dam in Iraq, and swept the peshmerga from four towns near Irbil, sparking a huge panic-driven flight of civilians, who’ve swelled the tens of thousands of Syrian refugees who fled to Iraqi Kurdistan.

Prothero, a McClatchy special correspondent, reported from Kalak, Landay from Washington. Contributing to this story was Adam Ashton of The News Tribune of Tacoma, Wash., in Baghdad, and Anita Kumar, James Rosen and Nancy A. Youssef in Washington.