Sunday saw widespread violence in Iraq
The number of United States military personnel killed in Iraq since the US-led invasion five years ago has passed the 4,000 mark.
The latest to die were four soldiers whose patrol vehicle was blown up by a bomb in southern Baghdad on Sunday.
President George W Bush offered his "deepest sympathies" to the families of US military personnel killed in Iraq.
Separately, the bodies of two US security contractors kidnapped in Iraq more than a year ago have been found.
The Federal Bureau of Investigation said it had recovered the remains of Ronald Withrow, who was seized in January 2007, and John Roy Young, abducted in November 2006.
On Sunday, insurgent attacks and military operations left at least 47 people dead across Iraq.
The bloodshed comes despite an overall reduction in violence since last June, following the US deployment of an extra 30,000 troops in violence-hit areas - the so-called "troop surge".
'Tragedies do happen'
According to an Associated Press breakdown of the 4,000 figure, 97% of deaths occurred after President Bush declared on 1 May 2003 that major combat was over.
Roadside bombs accounted for 44% of deaths last year and 55% to date in 2008, the agency adds.
US soldiers interviewed by AFP news agency in Iraq said they were saddened by the figure of 4,000, but argued the conflict was justified.
"Every one of those people signed up voluntarily and it's a shame that that happens, but tragedies do happen in war," said Senior Airman Preston Reeves, 26.
Staff Sgt Jonathan Criss, 37, said the US could not "just walk away and leave the Iraqi people".
"If you interviewed the 30,000 [wounded], they would have no ill will," he added.
The 4,000th death comes just days after Mr Bush marked the fifth anniversary of the invasion, saying that it had made the world a better place.
He said in his speech that the troop surge had "opened the door to a major strategic victory in the broader war on terror".
Reacting on Monday to the deaths of the four US soldiers, President Bush said: "I offer our deepest sympathies to their families."
He vowed "to make sure that those lives were not lost in vain" on what he described as a "day of reflection" to honour US war dead.
President Bush has repeatedly rejected any possibility of a full withdrawal until Iraq is able to defend itself.
He has the backing of the expected Republican candidate in this November's presidential election, Senator John McCain.
But Democratic hopefuls Barack Obama and Hillary Clinton have both pledged to end the war.
Senator Clinton has said she will start to withdraw troops within 60 days of being elected.
Senator Obama also favours a phased withdrawal, and has frequently said he would like all but a small residual force out within 16 months.