The main Sunni Arab political bloc withdrew from the Iraqi Cabinet on Wednesday, plunging the government into crisis on a day when suicide bombers killed more than 70 people with massive strikes in the capital.
The Sunni Accordance Front said that its five Cabinet members and Deputy Prime Minister Salam al-Zobaie would resign from the government.
"This is probably the most serious political crisis we have faced since the passage of the Constitution. If unresolved the implications are grave," the remaining deputy prime minister, Barhim Salih, a Kurd, said.
The Iraqi government said 1,653 civilians were killed last month, one-third more than the previous month, despite a fall in the number of deaths among US troops.
Fifty of Wednesday's dead were killed when a suicide bomber in a fuel truck packed with explosives targeted motorists at a petrol station, police said. Another suicide bomber killed 20 people outside a popular ice cream shop across town. Another bomber killed three in southern Doura district.
The Accordance Front said it was quitting Maliki's coalition because he had failed to meet about a dozen demands, including granting the Sunni bloc a greater say in security matters. Those standing down include the ministers of culture, women, planning, and higher education, and the junior foreign affairs minister.
Their withdrawal may have little practical effect on a government already paralyzed by infighting. The Shiite bloc of radical cleric Moqtada al-Sadr withdrew in April.
But the withdrawal was a blow to reconciliation efforts: luring the large Sunni bloc into government had been hailed as a major achievement when Maliki took power last year.
The US had hoped that the inclusion of Sunni Arabs in the Shiite-dominated government would reduce sectarian violence. But laws aimed at reconciliation have not been passed.
In a 45 minute conversation, US President George W. Bush prodded Maliki to unite rival factions, while the White House played down the significance of the Sunni bloc leaving the Iraqi government.
White House spokesman Tony Snow said that reconciliation was continuing.
Snow said that Vice President Tariq al-Hashemi and the minister of defense, both Sunnis, remained in place.
"There's political wrangling in Baghdad as there is in Washington," Snow said.
"The prime minister has made it clear that he remains determined to try to pull the coalition together," he said.
In the US Congress, Democrats and many Republicans have grown impatient with the Iraqi's government progress as US forces continue to fight and die.
Bush and al-Maliki noted some upbeat news on two other fronts.
They welcomed Saudi Arabia's pledge on Wednesday to open diplomatic relations with the Shiite-led government and to consider opening an embassy in Baghdad.
Bush also praised the Iraqi national team for winning the Asian Cup soccer tournament.
"Both leaders noted that the victory demonstrated to the world what Iraqis could do when they work together," Snow said.