Leaders of Iraq's main political blocs began a round of marathon meetings Tuesday with the U.S. ambassador in an attempt to reach agreement on a broad-based government.

Adbul-Aziz al-Hakim, leader of the main Shiite bloc, hosted the meeting, which was also attended by Kurdish, Sunni and secular leaders.

Iraq has been headed by a caretaker government since Dec. 15 parliamentary elections, and officials fear the vacuum in authority has contributed to surges of sectarian killing.

The new parliament is scheduled to meet for the first time Thursday, setting in motion a 60-day deadline for the legislature to elect a new president, approve the nomination of a prime minister and sign off on his Cabinet.

Leaders of the main ethnic and religious blocs, however, have been unable to agree on key issues, including how many positions various groups will have in the government, who will fill key posts and the government's program of action.

Among the most contentious is Shiite Prime Minister Ibrahim al-Jaafari's candidacy for a second term. Kurdish, Sunni and some secular leaders argue he is too divisive a figure and accuse him of doing too little to contain a wave of reprisal violence triggered by the Feb. 22 bombing of a revered Shiite shrine in Samarra.

The Shiite United Iraqi Alliance is itself divided over al-Jaafari. He won the nomination by just one vote last month in large part because of the support of radical Shiite cleric Muqtada al-Sadr. Al-Hakim favored Adil Abdul-Mahdi, one of two current vice presidents.

The stakes are high for the United States, which hopes the formation of an inclusive government would help stabilize Iraq so U.S. forces can start drawing down in the summer.

U.S. Ambassador Zalmay Khalilzad has been shuttling between the main Iraqi factions in a bid to reinvigorate negotiations that have dragged on for nearly three months.

Leaders of the main blocs agreed at a meeting with Khalilzad on Sunday to move parliament's inaugural session forward by three days in part to show their resolve to break the deadlock.

Also present at Tuesday's meeting were President Jalal Talabani and Massoud Barzani, leaders of the main Kurdish parties; Dhafir al-Ani, an official with the main Sunni bloc; and former Prime Minister Ayad Allawi, a secular Shiite.