The U.S. ambassador to Iraq pressed the country's leaders on Friday to end a political deadlock that has stalled formation of a grand coalition Washington sees as its best hope for a troop withdrawal.

As the Shi'ite Alliance said it would not succumb to pressure from Kurds, Sunnis and others to drop its nomination for prime minister, Zalmay Khalilzad called for a special conference of leaders from all sides to break the stalemate.

President Jalal Talabani convened the new parliament for March 19, a week beyond a supposed constitutional deadline, after a request from Shi'ites to give political parties more time to agree on key posts, including the assembly's speaker.

"We ... are focusing on a national unity government because without it Iraq would face disaster," he told Al Arabiya TV. "To avoid such a disaster, all our efforts are aimed at getting all sides to participate in a national unity government."

A suicide truck bomb killed at least 12 people near Falluja, including a U.S. Marine, police said, though the U.S. military put the toll at five. In Baghdad, a roadside bomb set ablaze a U.S. Abrams tank.

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