Ten US Marines were killed by an Iraqi bomb in one of the bloodiest incidents of the war for Americans, a day after President George W. Bush laid out a strategy he said would defeat the insurgency.
Yesterday's attack, two weeks before Iraqis vote for a new parliament, struck a foot patrol near Falluja.
Eleven Marines were wounded by an improvised explosive device (IED), the US military said this afternoon.
"The patrol was attacked with an IED fashioned from several large artillery shells," the Marines said.
Seven of the wounded had returned to active duty.
US commanders have expressed concern in recent months at the increasing use of more powerful and sophisticated roadside bombs. The high death toll yesterday indicated an extremely powerful blast.
Typically, US troops keep themselves well spaced out when on foot patrols to avoid the risk of mass casualties.
Local officials in Falluja said they were aware of a bomb attack on US troops overnight near Amiriya, 30 km (20 miles) south of Falluja.
US officials declined further comment, however, and it was unclear if this was the same incident.
In August, 14 Marines were killed when a landmine destroyed their armoured vehicle near Haditha in western Iraq.
Other heavy tolls have been inflicted on Americans when helicopters have been brought down and when a suicide bomber devastated a US army mess hall last December in Mosul, killing 21 people.
Falluja was the site of the biggest battle since US forces toppled Saddam Hussein in April 2003 - dozens of troops and hundreds of Iraqis were killed in the city, 50 km (30 miles) west of Baghdad in November last year.