The US has launched an air strike against militants from the Islamic State (IS) group in northern Iraq.
The Pentagon said its aircraft attacked artillery being used against Kurdish forces defending the city of Irbil.
The Sunni Muslim group IS, formerly known as Isis, now has control of large swathes of Iraq and Syria.
Tens of thousands of people from minority groups have fled their homes due to the militants' advance. IS has also seized Iraq's largest dam.
According to US officials, the dam is a vital part of Iraq's infrastructure as it controls the level of the Tigris River and is a key source of water and electricity generation for the Iraqi people.
According to the Pentagon statement, two F/A-18 planes from an aircraft carrier in the Gulf dropped 500-pound laser-guided bombs on mobile artillery near Irbil, where US personnel are based.
The air strike is the first time the US has been directly involved in a military operation in Iraq since American troops withdrew in late 2011.
No time limit has been set for air strikes, White House spokesman Josh Earnest told reporters, but he insisted that a "prolonged military conflict that includes US involvement is not on the table."
US Secretary of State John Kerry said the world needed to wake up to the threat posed by the IS group.
Its "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," he said.
IS fighters seized Qaraqosh, Iraq's biggest Christian town, earlier this week, causing many thousands to flee.
The militants' advance has also forced tens of thousands of Yazidis to leave their homes and escape to a nearby mountain.
'Carefully and responsibly'
Iraq's human rights ministry says the militants have seized hundreds of Yazidi women.
Ministry spokesman Kamil Amin, quoted by the Associated Press, said the women were below the age of 35 and some were being held in schools in Iraq's second largest city Mosul. He said the information had come from the captives' families.
There has been no independent confirmation of the report.
On Thursday, President Barack Obama said "America is coming to help" the people of Iraq.
He accused IS fighters of attempting the systematic destruction of entire populations.
At the same time, he announced that US military planes had already carried out air drops of food and water, at the request of the Iraqi government, to the many displaced Yazidis.
A statement from the US Defense Department later said planes had dropped 72 bundles of supplies, including 8,000 ready-to-eat meals and thousands of gallons of drinking water near Sinjar.
Britain and France have also pledged humanitarian support, with the UK sending £8m ($13.5m) of emergency aid.
The United Nations says it is working on opening a humanitarian corridor in northern Iraq to allow stranded people to flee.
Analysis: Jonathan Marcus, BBC defence correspondent
Rapid intervention in the north was a special case merited by the unfolding humanitarian catastrophe, but how much more support might the Americans give to the Kurds?
Amid all the talk of centrifugal forces in Iraq, the Kurdish north has the greatest degree of autonomy - some would argue it is already half-way out of the door of the Iraqi state.
Such a move would have huge implications for Turkey, Syria and Iran who all have Kurdish minorities.
Marzio Babille, Unicef's representative in Iraq, said the Yazidis were in an extremely precarious situation because of the "very aggressive and brutal" IS militants.
He said there were many "logistical and strategic difficulties", but added that a humanitarian corridor needed to be established.
Meanwhile all US airlines and a growing list of other carriers are not flying over Iraq due to the situation.
Back in June, when Isis took over Mosul, Iraqi Prime Minister Nouri Maliki requested US air strikes to halt the militants' advance - but Washington did not intervene.
Analysts say the relentless advance of IS fighters, together with the continuing failure of Iraqi politicians to agree on a new government, after an inconclusive election in April, may have swayed Mr Obama into deciding to act now.
Mr Maliki has faced calls from Sunni Arab, Kurdish and some Shia Arab leaders to step down because of his handling of the crisis.
But as leader of the bloc that won the most seats in April's parliamentary elections, Mr Maliki has demanded the right to attempt to form a governing coalition.
- The majority are Chaldeans, part of the Catholic Church
- Numbers have fallen from around 1.5 million since the US-led invasion in 2003 to 350,000-450,000
- In Nineveh, they live mainly in towns such as Qaraqosh (also known as Baghdida), Bartella, Al-Hamdaniya and Tel Kef
- Secretive group whose origins and ethnicity are subject to continuing debate
- Religion incorporates elements of many faiths, including Zoroastrianism
- Many Muslims and other groups view Yazidis as devil worshippers
- There are estimated to be around 500,000 Yazidis worldwide, most living in Iraq's Nineveh plains