Iraq conflict: US steps up air strikes on IS militants
The US has launched two additional air strikes against militants from the Islamic State (IS) group in northern Iraq, the Pentagon has said.
US drones and Navy fighter jets attacked targets near the Kurdish city of Irbil - a similar location to the first strike earlier on Friday.
The Sunni Muslim group IS has taken control of swathes of Iraq and Syria.
Tens of thousands of people from minority groups have fled their homes due to the militants' advance.
IS, formerly known as Isis, has also seized Iraq's largest dam.
These air strikes are the first time the US has been directly involved in a military operation in Iraq since American troops withdrew in late 2011.
The US later also confirmed a second air-drop of supplies to displaced people in the Sinjar area of northern Iraq.
In the second wave of air strikes, US drones destroyed a mortar position and killed a group of militants, the Pentagon said.
Analysis by BBC Washington correspondent David Willis
President Barack Obama hasn't set a timetable for the current intervention and White House spokesman Josh Earnest said simply that the length of the campaign would be determined by events on the ground.
He said that, ultimately, a solution to the current violence rests with the formation of an Iraqi government that is representative of all the Iraqi people. Yet the course of this engagement is difficult to predict.
White House officials concede they are concerned about the Islamic State fighters' proficiency - they are said to be well armed and well trained - hence it remains to be seen whether air strikes will be effective.
All of which raises the prospect of "mission creep" if the current campaign doesn't halt their advance.
Then just over an hour later, F/A-18 jets used laser-guided bombs to hit a seven-vehicle convoy of IS vehicles, spokesman Rear Admiral John Kirby said.
Earlier on Friday, two 500lb (227kg) bombs were dropped on IS artillery being used against forces defending Irbil, capital of the autonomous Kurdistan Region.
Marie Harf, a spokeswoman for the US state department, told the BBC that the immediate goal of the strikes was to "prevent the advance" of IS towards Irbil.
"Then longer term obviously we want to work to provide some time and space for the Kurdish forces to get back on their feet and fight this threat on their own," she said.
"There's not really a long term US military solution here."
IS recently seized towns held by Kurdish "Peshmerga" forces, forcing tens of thousands of people to flee into the mountains.
Although the Iraqi government and the Kurdistan Region have been at odds for months, Prime Minister Nouri Maliki sent a plane load of ammunition to Irbil on Friday, Reuters reported.
Mr Maliki has previously ordered the air force to support Kurdish fighters in their fight against IS.
The White House said Vice-President Joe Biden had called Iraqi President Fuad Masum on Friday and reiterated President Obama's commitment to help Iraqi civilians and bolster Iraq's ability to fight IS forces.
Earlier, US Secretary of State John Kerry said the world needed to wake up to the threat posed by the IS group.
He said its "campaign of terror against the innocent, including the Yazidi and Christian minorities" showed "all the warning signs of genocide".
British government sources told the BBC the air strikes were entirely "a US operation" and the UK was currently focused on humanitarian efforts.
While future participation in air strikes had not be ruled out, it had not been discussed by the UK government's emergency committee Cobra on Friday, and would not be over the weekend, the sources said.
IS fighters seized Qaraqosh, Iraq's biggest Christian town this week, causing thousands to flee. The militants' advance also forced tens of thousands of Yazidis to escape to mountains near Sinjar.
Late on Thursday, US military planes dropped food and water to help the displaced Yazidis and early on Saturday confirmed a second air-drop.
Iraq's human rights ministry says the militants have seized hundreds of Yazidi women.
Ministry spokesman Kamil Amin, quoted by AP news agency, said some were being held in schools in Iraq's second largest city Mosul.
Back in June, when Isis took over Mosul, Mr 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 election in April, may have swayed Mr Obama.
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, he 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