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 has called on Iraqis to unite and warned sectarian violence could destroy the country.
Grand Ayatollah Ali al-Sistani urged religious and political leaders, tribal chiefs and others to exert maximum efforts to stop the killing.
The Ayatollah's statement followed similar appeals for an end to the violence from the United States and the United Nations.
The Iraqi government reported Thursday that about 30,000 people have fled their homes to escape violence in the last month.
In other news, the United States says American and Iraqi soldiers have entered two cities Hawija and Riyadh in northern Iraq in search of suspected al-Qaida militants.
The operation follows terror attacks in the region, which killed 31 Iraqi soldiers.
Also Thursday, the U.S. military said a Marine was killed in fighting in al-Anbar province.
Some information for this report was provided by AP, AFP and Reuters.