Also on Friday morning, three mortar rounds exploded near the Abu Hanifa mosque in Baghdad. The mosque is one of the most important religious sites for Sunni Muslims in Iraq. A guard was wounded.
As political leaders on all sides pleaded for restraint and imposed a curfew on the capital, gunmen stormed a Sunni neighbourhood, burning four mosques and homes, an Interior Ministry official said.
The official said the number of casualties was not known, but a resident of Hurriya district, Imad al-Din al-Hashemi, said at least 18 people had been killed and 24 wounded.
"They attacked four mosques with rocket-propelled grenades and machinegun fire. The attacks began at midday," said Hashemi, who was helping to evacuate people from their homes.
The UN said on Wednesday that 3,709 civilians had been killed in Iraq in October, the most in any month since the war began 44 months ago. The figure is likely to be eclipsed in November.
The UN said citizens were leaving the country at 100,000 a month, and that at least 1.6 million Iraqis have left since the US invaded in March 2003.
The International Organisation for Migration, a UN-associated group, said on Tuesday that the number of Iraqis displaced by violence since the Samarra bombing in February 2006 has now increased to almost 250,000 in the 15 central and southern governorates, with more than 1,000 people on average being displaced a day in September, October and November.