President George W Bush has said US troops will stay in Iraq to complete their mission, following the death of 14 US marines and their interpreter.
Pacifying western Iraq is a priority for US-led forces
The roadside bombing in which they were killed was one of the deadliest attacks on US forces since the 2003 invasion.
It happened near the north-western city of Haditha, in the same area where seven marines were killed on Monday.
Washington is worried such strikes could affect the public mood in the US, the BBC's Adam Brookes says.
The latest attacks brought the US death toll to more than 1,800 since the Iraq invasion. More than 13,000 US troops have been wounded.
Polls in the US show a fairly constant level of pessimism among Americans at the prospects for a successful outcome in Iraq, our Pentagon correspondent says.
Privately, senior US officers say their greatest concern is the effect on public opinion in the United States of the deaths of American troops: If we lose America, said one general, we lose the war.
"We're at war. We're facing an enemy that is ruthless. If we put out a (pullout) timetable the enemy would adjust their tactics," Mr Bush said in a speech in Texas.
WORST US LOSSES IN IRAQ
31 killed in helicopter crash near Jordan border, Jan 05
17 die when two Black Hawk helicopters collide near Mosul, Nov 03
16 killed when Chinook shot down near Falluja, Nov 03
14 killed in attack on Mosul mess tent, Dec 04
14 killed in roadside bomb in Haditha, Aug 05
"They want us to retreat. They will fail. They do not understand the character and the strength of the United States of America," the president said.
He added that the best way to honour the US dead was to complete the mission in Iraq.
In the face of rising casualty figures, the Bush administration is now portraying Iraq's political process as the true measure of the country's progress, our correspondent says.
He says Washington in particular stresses on the importance of the new constitution that is now being drafted, a referendum due in October and elections scheduled in December.
The 14 US marines were killed when their vehicle was hit by a roadside bomb south of Haditha, about 260km (160 miles) north-west of Baghdad.
A top US defence official said the deaths came as marines were trying to contain "a very lethal... adaptable enemy" along the Euphrates valley.
Brig Gen Carter Ham, deputy director for operations at the Joint Chiefs of Staff, said missions were under way in a number of towns simultaneously to deny insurgents the ability to move around freely.
The Pentagon said it had no reason to believe Iraqi insurgents' claims that a marine was captured in an incident earlier in the week.
One of Iraq's most violent Islamic militant groups, Ansar al-Sunna, has claimed responsibility for an attack on Monday, saying it killed eight US marines and captured one serviceman.
The group said it had shot some of the troops and "slit the throats" of others. The web-posted claim cannot be verified.
At least 37 US military personnel have been killed in Iraq in the last 10 days, a period of intense violence, but the latest Haditha attack ranks among the biggest US losses.
Last December, 14 US troops and four civilian contractors died in a suicide bombing targeting a military base in Mosul.
Only air crashes have resulted in higher US death tolls, including 16 in the November 2003 loss of a Chinook helicopter near Falluja and 31 in a helicopter crash in January 2005 near the Jordanian border.