A conference to reconcile Iraq's warring political groups began to unravel even before it got under way on Tuesday, with the main Sunni Muslim Arab bloc pulling out and protesting it had not been properly invited.

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, a Shi'ite bloc and smaller groups underlined that while Iraq is no longer on the verge of a sectarian civil war, there are still deep divisions between Shi'ites and Sunnis, Iraq's main Muslim sects.

Sunni Arabs, a minority in Iraq, are upset that long-held grievances remain largely unanswered. Dominant under Saddam Hussein, they now complain of marginalisation and want greater political representation, more say in security matters and the release of thousands of Sunnis held in Iraqi prisons.

Iraqi Prime Minister Nuri al-Maliki gave an upbeat assessment despite Tuesday's setbacks, saying the nation was now healed after sectarian violence that threatened civil war.

"Peace has been achieved between Sunni and Shi'ite ... the crisis when people were being killed because of their religious identity is over," Maliki told the conference.

Washington has urged Iraq's leaders to match improvements in national security with political progress on legislation meant to reconcile majority Shi'ites and Sunni Muslims.

The Accordance Front pulled out of Maliki's Shi'ite-led government of national unity last August over its demands, plunging the government into crisis.

That withdrawal was followed by walkouts from other blocs, severely hampering the government's ability to pass legislation set by Washington as benchmarks in promoting reconciliation.

On Tuesday, Front spokesman Salim al-Jubouri complained that the bloc had not been properly invited to the reconciliation conference at a convention centre in Baghdad's heavily fortified "Green Zone", which houses the government and the US embassy.

He also said the Front decided not to attend because resolutions from other past conferences had not been acted upon. "How can we now arrange new proposals?" Jubouri said.

Politicians loyal to anti-US Shi'ite cleric Moqtada al-Sadr also pulled out of the government last year over its refusal to set a timetable for the withdrawal of US troops.

The parliamentary leader of the Sadrist bloc, Nassar al-Rubaie, attended the conference's opening, but the Sadrists later pulled out.

"Such conferences are just government propaganda," Rubaie told Reuters.

Almost all of Iraq's major political groups had said they would take part in the conference, many expressing hope progress would be made this time because of the security gains.

In opening remarks to the conference, Maliki criticised those who had stayed away. "We seriously regret that others will only stand watching, and others try to bring down the political progress and hamper the work of the government at a time when their patriotic duty requires them to help the government," Maliki said. - Reuters