Militiamen grabbed six Sunnis as they left Friday worship services, doused them with kerosene and burned them alive as Iraqi soldiers stood by, and seven Sunni mosques came under attack as Shiites took revenge for the slaughter of at least 215 people in the Sadr City slum.

A U.S. helicopter opened fire into the Shiite enclave after militiamen fired on it from the ground, residents said. There were no immediate reports of casualties.

With the government trying to avert a civil war, two simultaneous bombings in Tal Afar, in northern Iraq, killed at least 23 people. On Thursday, Sunni-Arab insurgents unleashed bombings and mortar attacks in Sadr City, the deadliest assault since the U.S.-led invasion.

Members of the Mahdi Army militia burned four mosques and several homes while killing 12 other Sunni residents in the once-mixed Hurriyah neighborhood until American forces arrived, said police Capt. Jamil Hussein. Gunmen loyal to radical anti-American Shiite cleric Muqtada al-Sadr began taking over the neighborhood this summer and a majority of its Sunni residents already had fled.

The gunmen attacked the four mosques with rocket-propelled grenades, machine guns and automatic rifles. Residents said the militiamen prevented them from entering the burned buildings to remove the dead, and they and Hussein said Shiite-dominated police and Iraqi military stood idly by.

Later Friday, militiamen raided al-Samarraie Sunni mosque in the el-Amel district and killed two guards, police 1st. Lt. Maitham Abdul-Razaq said. Two other Sunni mosques in west Baghad also were attacked, police said.

In Baghdad, followers of the radical Shiite cleric Muqtada al-Sadr warned they would suspend their membership in parliament and the Cabinet if Prime Minister Nouri al-Maliki meets with President Bush in Jordan next week, a member of parliament said. Bush and al-Maliki were scheduled to meet Wednesday and Thursday in Amman.

The al-Sadr bloc in parliament and government is the backbone of al-Maliki's political support, and its withdrawal, if only temporarily, would be a severe blow to the prime minister's already shaky hold on power.