BAGHDAD >> Clashes between Iraqi troops and Sunni militants west of Baghdad killed at least four children on Thursday as the United Nations announced its highest level of emergency for the humanitarian crisis ignited by the advance of Islamic militants across much of the country’s north and west.

The extremist Islamic State group’s lightning advance has driven hundreds of thousands of people from their homes since June, and last week prompted the U.S. to launch aid operations and airstrikes as the militants threatened religious minorities and the largely autonomous Kurdish region.

The U.N. on Wednesday declared the situation in Iraq a “Level 3 Emergency” — a development that will allow for additional assets to respond to the needs of the displaced, said U.N. special representative Nickolay Mladenov, pointing to the “scale and complexity of the current humanitarian catastrophe.”

The Security Council also said it was backing a newly nominated premier-designate in the hope that he can swiftly form an “inclusive government” that could counter the insurgent threat, which has plunged Iraq into its worst crisis since U.S. troops withdrew at the end of 2011.

The U.N. move came after some 45,000 people, members of the Yazidi religious minority, were able to escape from a remote desert mountaintop where they had been encircled by Islamic State fighters, who view them as apostates and had vowed to kill any who did not convert to Islam.

They were able to reach safety after Kurdish fighters from neighboring Syria opened an exit corridor. U.S. and Iraqi forces had earlier airlifted aid to those trapped.

U.S. officials said Thursday that roughly 4,500 people remain on Sinjar Mountain, nearly half of whom are herders who lived there before the siege and have no desire to leave. The two officials spoke on condition of anonymity because they weren’t authorized to speak publicly.

The U.N. said it would provide increased support to the Yazidis and to 400,000 other Iraqis who have fled since June to the Kurdish province of Dahuk. A total of 1.5 million people have been displaced by the fighting.

The United States has been carrying out airstrikes in recent days against Islamic State fighters, helping fend back their advance on Kurdish regions.

In western Iraq, fighting erupted early on Thursday in the militant-held city of Fallujah, about 40 miles west of Baghdad. The clashes on the city’s northern outskirts killed four children, along with a woman and at least 10 militants, said Fallujah hospital director Ahmed Shami.

He had no further details on the clashes, beyond saying that four other children and another woman were wounded in the violence.

It was difficult to gauge the situation in Fallujah, which has been in the hands of the Islamic State and allied militants since early January, when the insurgents seized much of the western Anbar province along with parts of the provincial capital of Ramadi.

Meanwhile, eight civilians were killed in separate attacks across Baghdad on Thursday.

A bomb attached to a minibus in the central Sheik Omar area killed four commuters and wounded 11 others, a police officer said. Another bomb went off in a commercial area in the southeastern Bayaa neighborhood, killing two and wounding nine, another police officer said.

Two other civilians were killed and 11 wounded when two mortar rounds struck another residential area, he said.

Three medical officials confirmed the casualty figures. All officials spoke on condition of anonymity as they were not authorized to brief media.

Iraq’s leaders have been struggling to agree on a new government that can address the crisis, with mounting calls for Prime Minister Nouri al-Maliki to step down despite his bloc winning the most votes in April’s election.

Iraq’s president has nominated a Shiite politician, Haider al-Abadi, to form the next government, putting him on track to replace al-Maliki.

But al-Maliki said Wednesday he will not relinquish power until a federal court rules on what he called the “constitutional violation” by President Fouad Massoum.

Al-Maliki insists he should have a third four-year term but appears increasingly isolated as many of his erstwhile Shiite allies and the international community have lined up behind al-Abadi, who has 30 days to propose a Cabinet.

The U.N. Security Council urged al-Abadi to work swiftly to form “an inclusive government that represents all segments of the Iraqi population and that contributes to finding a viable and sustainable solution to the country’s current challenges.”