President George W Bush has declared that US resolve in Iraq remains "unshakable", despite ongoing clashes with insurgents across the country.
US troops have been involved in the street-to-street fighting in Falluja
His comments came after one of the worst single attacks since the war left 12 US marines dead and about 20 others injured in Ramadi, west of Baghdad.
Tuesday's attack was one of several clashes between US-led troops and Shia and Sunni Muslim gunmen.
At least 36 Iraqis are said to have been killed in Falluja in the past day.
The last three days of clashes throughout the country are estimated to have left more than 100 Iraqis dead.
MAJOR US COMBAT LOSSES
6 April 2004: 12 marines killed and 20 injured at Ramadi
2 November 2003: helicopter shot down near Amiriya with 15 soldiers killed
23 March 2003: 29 soldiers killed at Nasiriya
During the same period, at least 20 coalition troops have died.
The Shia-led violence has opened a second front for US-led coalition troops who had previously been confronting mainly Sunni supporters of the former Iraqi leader, Saddam Hussein.
The action by the Shias was triggered by the closure of radical cleric Moqtada Sadr's al-Hawza newspaper a week ago on the grounds that it was inciting violence.
- Italian troops killed 15 Iraqis in clashes with Shia militants in the southern city of Nasiriya on Tuesday.
- US Defence Secretary Donald Rumsfeld says there are no plans to send additional troops to Iraq.
"The president mourns the loss of each of our fallen," White House spokesman Scott McClellan told reporters.
"Our resolve is firm... and we will prevail," the spokesman said after Mr Bush had been briefed about the attack at his ranch in Crawford, Texas.
He said the president would hear more about the fighting during a video link conference with his national security council later on Wednesday.
The White House is now back on a war footing, the BBC's Justin Webb in Washington reports, and Secretary of State Colin Powell has urged the nation to rally behind its troops.
In an unusual foray into domestic politics, Mr Powell criticised Democratic Senator Edward Kennedy for describing Iraq as George Bush's Vietnam.
Mr Powell said Senator Kennedy should be a little more restrained and careful in comments because America was at war.
Dozens of Iraqis attacked the marines near the Ramadi governor's palace and "a significant number" of them were killed, a Pentagon official said.
The offensive in Falluja has been going on since Monday in an effort to subdue the city after four Americans were killed and their bodies mutilated there last week.
US troops - using tanks and helicopter gunships - have been involved in the street-to-street fighting with insurgents armed with guns and rocket propelled grenades.
Ramadi and Falluja together with Tikrit are within the so-called "Sunni triangle" - a hotbed of anti-coalition activity.
But the clashes which are proving of greatest concern to the coalition have been with Shia militias loyal to Moqtada Sadr in a string of towns south of Baghdad, says the BBC's Richard Lister in Baghdad.
They had not previously been active, but have now taken on British, Spanish and Italian troops, as well as the Americans, and there have been scores of casualties, mostly Iraqis.
In Baghdad on Wednesday, tanks were still standing guard outside police stations in the poor neighbourhood of Sadr City.
An arrest warrant has been issued for Mr Sadr on charges unrelated to the current violence.
The cleric remains surrounded by armed supporters in the holy city of Najaf.
His militia, known as the Mehdi Army, has staged violent demonstrations and attacked US-led forces in several Iraqi cities and has vowed that the unrest will continue.