US President Barack Obama has said he will not allow jihadists to carve out a "caliphate" straddling Syria and Iraq.
On Friday, the US began launching air strikes on fighters of the Islamic State (IS) in northern Iraq.
Mr Obama said there would be further air strikes if necessary but no US military operation on the ground.
The UK has sent a plane to join the relief operation on Mt Sinjar, where thousands of members of the Yazidi religious minority are stranded.
A cargo plane left RAF Brize Norton in England carrying humanitarian aid.
The Pentagon earlier said it had carried out a second air drop that included more than 1,500 gallons of water and 28,000 meals.
About 50,000 Yazidis fled into the mountains after IS fighters overran the nearby town of Sinjar a week ago.
"We have one or two days left to help these people. After that they will start dying en masse," Yazidi MP Vian Dakhil told the AFP news agency.
IS, formerly known as Isis, has taken control of swathes of northern and western Iraq, and recently seized Mosul dam, the country's largest.
As well as threatening Yazidis and Christians, the group is confronting Kurdish Peshmerga forces and threatening the city of Irbil, capital of the semi-autonomous Kurdistan Region.
'We can run them off'
US fighter jets carried out two waves of air strikes west of Irbil on Friday - the first time US forces have been directly involved in a military operation in Iraq since they withdrew from the country in late 2011.
In an interview with the New York Times, Mr Obama said he was willing to consider broader use of military strikes to push back IS, but warned that Iraq's political leaders had to start working with each other.
"We're not going to let them create some caliphate through Syria and Iraq," he said. "But we can only do that if we know that we have got partners on the ground who are capable of filling the void."
Iraq's politicians have been unable to form a government since April's parliamentary elections, which were won by Prime Minister Nouri Maliki's State of Law Coalition.
He has rejected calls to step down from Sunni Arabs, Kurds and some fellow Shia Arabs, who accuse him of mishandling the security crisis and pursuing sectarian and authoritarian policies during his two terms in office.
In his weekly radio address, Mr Obama emphasised that the US air strikes and humanitarian effort in northern Iraq were vital but limited.
"In recent days, terrorist forces neared [Irbil]," he said. "I made it clear that if they attempted to advance further, our military would respond with targeted strikes.
"That's what we've done and, if necessary, that's what we will continue to do."
He also promised not to let the US be "dragged into fighting another war in Iraq".
A senior official in the Kurdistan Regional Government said that following the US air strikes, the Peshmerga would "first regroup, second redeploy in areas they retreated from, and third help the displaced return home".
- 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 province, 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