IRBIL, Iraq >> The U.S. unleashed its first airstrikes in northern Iraq against militants of the Islamic State group Friday amid a worsening humanitarian crisis. The extremists took captive hundreds of women from a religious minority, according to an Iraqi official, while thousands of other civilians fled in fear.

Many of America’s allies backed the U.S. intervention, pledging urgent steps to assist the legions of refugees and displaced people. Those in jeopardy included thousands of members of the Yazidi religious minority whose plight — trapped on a mountaintop by the militants — prompted the U.S. to airdrop crates of food and water to them.

The extremists’ “campaign of terror against the innocent, including the Yazidi and Christian minorities, and its grotesque and targeted acts of violence bear all the warning signs and hallmarks of genocide,” said U.S. Secretary of State John Kerry. “For anyone who needed a wake-up call, this is it.”

Underscoring the sense of alarm, a spokesman for Iraq’s human rights ministry said hundreds of Yazidi women had been seized by the militants. Kamil Amin, citing reports from the victims’ families, said some of the women were being held in schools in Iraq’s second-largest city, Mosul.

“We think that the terrorists by now consider them slaves and they have vicious plans for them,” Amin told The Associated Press. “We think that these women are going to be used in demeaning ways by those terrorists to satisfy their animalistic urges in a way that contradicts all the human and Islamic values.”

For the U.S. military, which withdrew its forces from Iraq in late 2011 after more than eight years of war, the re-engagement began when two F/A-18 jets dropped 500-pound bombs on a piece of artillery and the truck towing it. The Pentagon said the militants were using the artillery to shell Kurdish forces defending Irbil, the capital of Iraq’s autonomous Kurdish region, and home to a U.S. consulate and about three dozen U.S. military trainers.

Later Friday, the U.S. launched a second round of airstrikes near Irbil, U.S. officials said. The officials, speaking on condition of anonymity because they weren’t authorized to discuss the strikes publicly, said unmanned aircraft hit a mortar and four Navy F/A-18 fighter jets destroyed a seven-vehicle convoy

Expanding from their stronghold of Mosul, the militants have captured a string of towns and Iraq’s largest hydroelectric dam and reservoir in recent weeks. Ethnic and religious minorities, fearing persecution and slaughter, have fled as their towns fell.

Many had taken refuge in the Khazer Camp, set up near Irbil, but it was empty Friday as nearby fighting prompted families to flee once again.

Some made their way by car or on foot to Irbil; others were unaccounted for amid the sea of fleeing people. According to the U.N., more than 500,000 people have been displaced by the violence in Iraq since June, bringing the total this year to well over 1 million.

In Irbil, hundreds of uprooted men crowded the streets of a Christian-dominated neighborhood, expressing relief at the news of U.S. airstrikes.

Nazar, one man lingering outside a bare-bones building-turned-shelter, fled his mainly Christian town of Hamdaniya on Wednesday, when their home began to shudder from the blast of nearby mortar fire.

“We want a solution,” said Nazar, who spoke on condition he be identified only by his first name, fearing his family’s safety. “We don’t to flee our homes and jobs like this. What is our future?”

In contrast to Washington’s decision to invade Iraq more than a decade ago, both the airdrop and the authorization of military action against the Islamic State group were widely welcomed by Iraqi and Kurdish officials fearful of the militants’ advance.

“We thank Barack Obama,” said Khalid Jamal Alber, from the Religious Affairs Ministry in the Kurdish government.

In his announcement Thursday night, Obama had identified protecting the Yazidis and defending Americans as the two objectives for the airstrikes.

But on Friday, his spokesman, Josh Earnest, said the U.S. was also prepared to use military force to assist Iraqi forces and the Kurds’ peshmerga militia.

While Iraq’s military has proven unable in many cases to thwart the Islamic State force’s capture of key cities, Earnest called the peshmerga a “capable fighting force” that had shown an ability to regroup effectively.

At a checkpoint about 23 miles (38 kilometers) from Irbil, Kurdish militiamen vowed fierce resistance to any further Islamic State advances, but they also remarked on the ferocity of their foe.

Capt. Ziyran Mahmoud, 28, said Islamic State fighters wore suicide belts as they advanced in armored vehicles and would detonate them — killing soldiers from both sides — if Kurdish fighters came too close.

“They are ready to blow themselves up and die,” Mahmoud said. “But the peshmerga aren’t afraid. We are also ready to die for our homeland.”

The Islamic State group captured Mosul in June, and then launched a blitz toward the south, sweeping over Sunni-majority towns almost to the capital, Baghdad. It already holds large parts of western Iraq, as well as swaths of neighboring Syria.

Iraqi government forces crumbled in the face of the assault but have since been able to prevent the militants from advancing into Shiite-majority areas. In the north, the Kurds have been the main line of defense against the radicals, but their fighters are stretched over a long front trying to fend them off.

U.S. Defense Secretary Chuck Hagel, traveling in India, said if Islamic militants threaten U.S. interests in Iraq or the thousands of refugees in the mountains, the U.S. military has enough intelligence to clearly single out the attackers and launch effective airstrikes.

He also said more than 60 of the 72 bundles of food and water airdropped onto the mountain reached the people stranded there.

The International Rescue Committee said it was providing emergency medical care for up to 4,000 dehydrated Yazidis, mostly women and children, who survived without food or water for up to six days hiding in the Sinjar mountains before fleeing to a refugee camp in Syria, where a civil war is raging.

Officials in Britain, Germany and elsewhere pledged financial aid to support humanitarian efforts in Iraq, and several top European officials supported Obama’s decision to intervene with airstrikes.

British Prime Minister David Cameron expressed special concern for the Yazidis trapped on Mount Sinjar.

“They fear slaughter if they descend back down the slopes but face starvation and dehydration if they remain on the mountain,” Cameron said. The world must help them in their hour of desperate need.”

One Yazidi man, who identified himself as Mikey Hassan, said he, his two brothers and about a dozen other relatives fled into the Sinjar mountains and then managed to escape to the Kurdish province of Dohuk after two days by shooting their way past the militants. Hassan, in a telephone interview with the AP, said he and his family went about 17 hours with no food before getting some bread. He said there were a few wells in the mountains, but they were largely ineffective because throngs of people were using them.

Yazidis belong to ancient religion seen by the Islamic State group as heretical. The group also sees Shiite Muslims as apostates, and has demanded Christians either convert to Islam or pay a special tax.

Pope Francis also was engaged, sending an envoy to Iraq to show solidarity with Christians who have been forced from their homes. There also was a papal plea on Twitter: “Please take a moment to pray for all those who have been forced from their homes in Iraq.”

In response to the fighting, Lufthansa, Turkish Airlines and other carriers canceled flights to and from Irbil.

In the U.S., the FAA banned American carriers from flying over Iraq, saying hostilities there could threaten safety. British Airways also said it was temporarily suspending flights over Iraq

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Associated Press writers Robert Burns and Josh Lederman in Washington, Danica Kirka in London, Caleb Jones in New York, Nicole Winfield in Rome, Sameer N. Yacoub and Vivian Salama in Baghdad, and Lolita C. Baldor in New Delhi contributed to this report.