Shiite militiamen grabbed six Sunnis as they left worship services Friday, doused them with kerosene, and burned them alive, Iraq police said, making for a total of at least 25 Sunnis who were killed in a seemingly retaliatory move to Thursday's triple homicide bombing targeting Shitte that killed more than 200.

Members of the Mahdi Army military then set at least three mosques and several homes on fire, while Iraqi soldiers stood nearby and did not intervene, police Capt. Jamil Hussein said.

A U.S. helicopter opened fire into Sadr City after Shiite militiamen attacked it with heavy gunfire from the ground Friday night, residents of the Shiite enclave said.

At least 23 died in northern Iraq after a car laden with explosives and a homicide bomber wearing with explosives hidden in a belt exploded simultaneously outside a car dealership in Tal Afar. With 26 wounded, the death toll was expected to rise, said police Brig. Khalaf al-Jubouri.

As funeral processions were held in Sadr City on Friday, several mortar rounds hit the Um al-Qura mosque, headquarters of Association of Muslim Scholars in west Baghdad's Ghazaliyah neighborhood, wounding four of the guards, said police Capt. Jamil Hussein.

In Baghdad's mostly Shiite neighborhood of Hurriyah, clashes between Shiite militiamen and Sunni insurgents armed with machine guns and rocket-propelled grenades broke out near a Sunni mosque, residents said. No casualties were immediately reported in the fighting.

Three mortar rounds also exploded near the Abu Hanifa mosque, Sunni Islam's most important shrine in another area of Baghdad at 9:45 a.m. Friday, wounding one guard, said its sheik, Samir al-Obaidi. A mortar round crashed through the dome of the structure Thursday night, within hours of the Sadr City attack.

In the Shiite bastion, hundreds of men, women and children beat their chests, chanted and cried as they walked beside vehicles carrying the caskets of their loved ones.

The bloodshed underlined the impotence of the Iraqi army and police to quell determined sectarian extremists at a time when the United States appears to be considering a move to accelerate the handover of security responsibilities.

"We condemn such acts of senseless violence that are clearly aimed at undermining the Iraqi people's hopes for a peaceful and stable Iraq," said White House spokesman Jeanie Mamo.

U.N. Secretary-General Kofi Annan also condemned the violence and in a statement urged "the Iraqi people to heed the calls by political and religious leaders from all sides for calm and restraint to prevent an escalation of the situation."

Baghdad remained under a 24-hour curfew aimed at stopping revenge attacks. But al-Maliki, himself a Shiite, ordered police to guard the processions carrying victims of Thursday's attacks by Sunni Muslim insurgents for burial in Najaf, the holy Shiite city.

"God is great. There is no God but Allah. Muhammad is the messenger of Allah," about 300 mourners chanted as they beat their chests while walking through the Sadr City slum alongside slow moving the cars and minivans carrying 16 wooden caskets tied to the rooftops. Some of the men and women repeatedly touched the sides of the vehicles or the caskets in an effort to say a final farewell to their relatives or friends.

Once the processions reached the edge of Sadr City in northeastern Baghdad, the cars and minivans left most of the mourners behind and began the 100-mile drive south to Najaf, a treacherous journey that passes through many checkpoints and areas controlled by Sunni militants in Iraq's so-called "Triangle of Death."

As cleanup crews continued removing pieces of human flesh from wreckage of the car bomb attacks, tents were erected where the families of the dead could receive condolences from friends and relatives.

In addition to the curfew, Friday is a day of worship in mostly Muslim Iraq when many people have the day off work. For several months, the government has been imposing a 11 a.m. to 3 p.m. ban on vehicles on Fridays, forcing people to walk to their local mosques for services.

In the well-coordinated Sadr City attack, Sunni insurgents blew up five car bombs and fired mortars, forcing Iraqi leaders into a meeting aimed at containing the growing sectarian war.

The attack surpassed coordinated blasts on March 2, 2004, that struck Shiite Muslim shrines in Karbala and Baghdad, killing a total of at least 181 Iraqis and wounding 573. A bombing in the southern city of Hillah that targeted mostly Shiite police and National Guard recruits, killed 125 and wounded more than 140 in February 2004.

Shiite mortar teams quickly retaliated to the Sadr City onslaught on Thursday, firing 10 shells that badly damaged the Abu Hanifa mosque in the Azamiya neighborhood and killed one person.

The bloodshed underlined the impotence of the Iraqi army and police to quell determined sectarian extremists at a time when the United States appears to be considering a move to accelerate the hand-over of security responsibilities.

"We condemn such acts of senseless violence that are clearly aimed at undermining the Iraqi people's hopes for a peaceful and stable Iraq," White House spokesman Jeanie Mamo said in Washington.

On Thursday night, Iraq's government imposed the curfew in the capital and also closed its international airport to all commercial flights. The transport ministry then took the highly unusual step of closing the airport and docks in the southern city of Basra, the country's main outlet to the vital shipping lanes in the Gulf.

Leaders from Iraq's Shiite, Sunni and Kurdish communities issued a televised appeal for calm. Al-Maliki also went on state TV and blamed Sunni radicals and followers of Saddam Hussein for the attacks on Sadr City.

The coordinated car bombings — three by homicide drivers and two of parked cars — billowed black smoke up into clouds hanging low over blood-smeared streets jammed with twisted and charred cars and buses in the sprawling Shiite slum, which is a stronghold of the Mahdi Army militia of al-Sadr, a key al-Maliki backer.

The militia and associated death squads are believed responsible for the slayings of hundreds of Sunnis since suspected Al Qaeda in Iraq militants bombed a revered Shiite shrine in the city of Samarra last February. That attack set off a surge of retaliatory killings between Shiites and Sunnis that have raged all year.

The Sadr City slaughter occurred moments after an attack by 30 masked Sunni gunmen who tried to storm the Shiite-dominated Health Ministry, about a mile west of the Shiite slum. Seven ministry guards were wounded.

The Associated Press contributed to this report.