The U.S. unleashed its first airstrikes in northern Iraq against militants of the Islamic State (ISIS) 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.

Military aircraft also dropped food and water around the town of Sinjar and nearby mountains early Saturday local time, the Pentagon said in a statement. 

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."

More than 1 million displaced

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 Erbil, 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 Erbil, 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.

Iraq children Nineveh

The militant group Islamic State, which has declared a caliphate in parts of Iraq and Syria, has prompted tens of thousands of Yazidis and Christians to flee for their lives during their push to within a 30-minute drive of the Kurdish regional capital Arbil. (Reuters)

The U.S. State Department warned U.S. citizens against all but essential travel to Iraq and said those in the country were at high risk for kidnapping and terrorist violence.

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 Erbil, but it was empty Friday as nearby fighting prompted families to flee once again.

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

In Erbil, hundreds of uprooted men crowded the streets of a Christian-dominated neighbourhood, 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?"

'We thank Barack Obama'

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.

Iraq U.S. airstrikes

Airstrikes by the U.S. forces marked the first time U.S. forces have directly targeted the extremist Islamic State group known as ISIS, which controls large areas of Syria and Iraq. (Khalid Mohammed/The Associated Press)

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 38 kilometres from Erbil, 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 armoured 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."

Canada supports U.S. action

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 neighbouring Syria.

Obama US Iraq

The U.S. said Islamic rebels had been using the artillery to shell Kurdish forces defending Erbil where U.S. personnel are located. (Charles Dharapak/Associated Press)

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

U.S. Defence Secretary Chuck Hagel, travelling 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.

In a press release, Canadian Foreign Affairs Minister John Baird said the government supports U.S. military efforts, but has not yet received requests for military assistance.

Baird said Canadian officials, including the ambassador to Iraq who is currently based in Jordan, will determine how best to support Iraqis with the security and humanitarian challenges.

With files from CBC News