BAGHDAD — The U.N. urged Iraq’s leaders Saturday to overcome their deep divisions and move quickly to form a new government that can unite the country and confront a surging militant threat, warning that failure to do so “risks plunging the country into chaos.”

The Sunni insurgent blitz over the past month has driven Iraq into its deepest crisis since the last American troops left in 2011, pushing bloodshed to levels unseen since the height of the Iraq war, sending Sunni-Shiite tensions soaring and raising the specter of a nation cleaved in three along ethnic and sectarian lines.

Iraq’s new parliament is scheduled on Sunday to hold its second session amid hopes that lawmakers can quickly decide on a new prime minister, president and speaker of parliament — the first steps toward forming a new government. It failed to make any progress in its first session, and postponed its second session until Sunday.

U.N special envoy to Iraq, Nickolay Mladenov, called on lawmakers to attend the meeting and forge an agreement on new leaders. He warned of dire consequences if the current political deadlock drags on.

“It will only serve the interests of those who seek to divide the people of Iraq and destroy their chances for peace and prosperity,” he said in a statement. “Iraq needs a team that can bring people together. Now is not the time for mutual accusations, now is the time for moving forward and compromising in the interest of the Iraqi people.”

Prime Minister Nouri al-Maliki, who has ruled the country since 2006, is under pressure to step aside. His government’s inability to prevent the attack, let alone roll back the militant advance, has sapped public — and international — confidence in his ability to hold Iraq together and lift it out of the crisis.

Al-Maliki’s opponents, and even many of his former allies, accuse him of trying to monopolize power and alienating the Sunni community, and are pushing him to not seek a third consecutive term. Al-Maliki has so far refused to withdraw his candidacy, and points to his State of Law bloc’s capturing the most seats in April elections to claim he has a mandate.