A conference to reconcile Iraq's rival political parties fell apart almost as soon as it began on Tuesday, with influential Sunni and Shia blocs pulling out in protest.
Hundreds of politicians gathered for the conference a day after US Vice President Dick Cheney, on a visit marking the fifth anniversary of the March 2003 US-led invasion, hailed what he called "phenomenal" political and security improvements.
The war has cost the United States $500 billion since it began. Tens of thousands of Iraqis have been killed and millions displaced. Almost 4,000 US soldiers have also been killed in the war, a major issue in November's US presidential election.
The boycott by the Sunni Arab Accordance Front, the Shia bloc led by cleric Moqtada al-Sadr and smaller groups underlined that while Iraq is no longer on the verge of a sectarian civil war, there are still deep divisions between Iraq's main sects. Shia Prime Minister Nuri al-Maliki accused the groups of seeking to damage Iraq's reconciliation efforts.
Maliki is under pressure from Washington to reach political accommodation with opponents that will cement the fragile security gains. The gathering - held just two days before the fifth anniversary of the 2003 US-led invasion - was intended to give renewed impetus to the reconciliation.
US Republican presidential candidate John McCain, speaking in Jordan after visiting Iraq, said the recent build-up of US troops was succeeding and any early military withdrawal would dramatically boost Iranian influence in the region.
"If we pull out of Iraq ... then obviously the Iranian influence is dramatically increased, al Qaeda has greater influence and endangers the region dramatically, and the United States's image and security challenges are dramatically increased," he said.