A U.S. military spokesman says insurgents are streaming into Baghdad for what he calls an "all-out assault" on the country's capital.
Major General William Caldwell said Thursday attacks against U.S. and Iraqi forces in the capital are up sharply since last Friday. He said control of Baghdad is essential for the new government of Prime Minister Nouri al-Maliki.
Iraq's top Shi'ite Muslim cleric warned on Thursday the increasing sectarian violence could destroy the country and called on Iraqis to unite to stop the killings.
In a statement released on Thursday, Grand Ayatollah Ali al-Sistani urged religious and political leaders, tribal chiefs and others to exert maximum efforts to stop the violence.
Meanwhile, five people were killed when a car bomb exploded in the northern city of Kirkuk - bringing to at least 35 the number of people killed on Thursday across the country.