Sunni ministers are boycotting government business, and the deepening crisis has cast doubt on the US-backed regime's ability to push through reforms designed to reunite the war-torn country.

Washington has demanded that series of law and constitutional amendments be passed to appease Sunni resentment and end faction fighting, as the programme was supposed to proceed alongside a surge of US troops into the country.

On Saturday, the main Sunni bloc in the coalition reacted angrily to a rebuke from Prime Minister Nuri al-Maliki's spokesman, who had accused its ministers of disrupting legislation and had implied they were corrupt.

"The Iraqi Concord Front rejects and strongly condemns the statement of the official government spokesman and the lies and the insinuations and attacks on people known for their integrity," the Front said in a statement.

It came one day after Maliki's spokesman Ali al-Dabbagh launched his withering attack on the Sunni bloc, which holds 44 seats in parliament but has withdrawn its six ministers from the government.

"The politics of threats, of pressure, and of blackmail will not work, and crippling the work of the government and the parliament and the political process will not return Iraq to the days of dictatorship," Dabbagh said.

This situation "does not benefit the people of Iraq, but those who fill their bank accounts outside the country by benefitting from the politics of complaining, crying, and begging in the name of this sect or that," he added.

Meanwhile, a parked car bomb exploded in a busy shopping street in predominantly Shia eastern Baghdad on Saturday, killing at least four people and wounding 10, police said.

The bomb was the latest in a series of explosions targeting commercial centres.