Iraqi men fill military trucks to join the Iraqi army at the main recruiting center in Baghdad, Tuesday, after authorities urged them to help battle insurgents. Hundreds of young Iraqi men gripped by religious and nationalistic fervor streamed into volunteer centers across Baghdad Saturday, answering a call by the country’s top Shiite cleric to join the fight against Sunni militants advancing in the north.

BAGHDAD — Militants pushed a weeklong offensive that has overrun swathes of Iraq to within 60 km of Baghdad Tuesday, as the UN warned the country’s very existence was under threat.

Washington, meanwhile, deployed extra troops to protect its embassy in Baghdad and was mulling air strikes against militants whose swift assault has seen them capture Mosul, a city of two million people, and subsequently a vast amount of territory north of Baghdad.

Iraqi officials have insisted they are making progress in repelling the fighters, who are led by the powerful Islamic State of Iraq and the Levant (ISIL), but have continued to lose ground.

The crisis has displaced hundreds of thousands of people and sparked fears that the violence could impact the country’s vast oil production, along with concerns that security forces won’t be able to halt the insurgents’ march on the capital.

Officials said Tuesday that the fighters briefly held areas of Baquba, a short drive from Baghdad, and took control of most of Tal Afar, a strategic Shiite-majority town in north Iraq that lies along a crucial corridor to Syria.

In Tal Afar, militants were controlling most of the town but pockets of resistance remained, and soldiers, policemen and armed residents held on to parts of its airport, according to Nineveh provincial council deputy chief Nureddin Qabalan.

The swift advance of the militants has sparked international alarm, with UN envoy to Baghdad Nickolay Mladenov warning that Iraq’s sovereignty is at stake.

“Right now, it’s life-threatening for Iraq but it poses a serious danger to the region,” Mladenov told AFP.

“Iraq faces the biggest threat to its sovereignty and territorial integrity” in years, he added.