US President George W Bush expressed sorrow on Monday as the number of US troops killed in Iraq hit 4,000, days after the fifth anniversary of the conflict when he predicted eventual victory.
"One day people will look back at this moment in history and say 'thank God there were courageous people willing to serve' because they laid the foundation for peace for generations to come," Bush said after a roadside bomb killed four US soldiers, pushing the toll to the new milestone.
Democratic presidential contenders Barack Obama and Hillary Clinton praised the heroism of US troops while promising that if elected they would come home.
Obama said the war should never have been waged and troops should be brought home soon. Clinton pledged to respond "by bringing a responsible end to this war, and bringing our troops home safely."
After a State Department briefing with Secretary of State Condoleezza Rice, Bush offered words of comfort for the families of the fallen.
"I hope their families know that citizens pray for their comfort and strength, whether they were the first one who lost their life in Iraq or recently lost their lives in Iraq," said.
White House spokeswoman Dana Perino described the milestone as "a sober moment, and one that all of us can focus on in terms of the number 4,000."
Pentagon spokesman Bryan Whitman said: "Every single loss of a soldier, sailor, airman and Marine is keenly felt by us in the department, by military commanders, by families, friends, both in theater and here at home."
Precise Iraqi casualties in the conflict are not known but the widely cited human rights group Iraq Body Count said earlier this month that up to around 89,300 civilians have been killed since 2003.
Recent public opinion polls show around 30 percent of voters approve of Bush's handling of the war and roughly the same number believe the loss of American life was worthwhile.
The 4,000th US death came days after Bush marked the fifth anniversary of the war and said the United States was on track toward victory.
The president chaired a meeting of his National Security Council on Monday and was briefed by Ryan Crocker, the U.S. ambassador in Baghdad, and General David Petraeus, the top US military officer in Iraq.
Bush and his advisors are trying to decide whether to continue to reduce troop levels in Iraq after last year's surge, which is credited with lowering violence there.
Some experts are urging a pause in troop reductions to avoid losing the gains made in recent months.
Perino said Bush wanted to be sure the gains made in the past years were cemented. The aim was to "lay the foundation for Iraq to have a democracy where they can govern, sustain and defend itself."
The deaths that pushed the US toll in Iraq to 4,000 happened as new violence burst out, including sustained mortar fire against the US protected "Green Zone" in Baghdad.
Whitman said that despite the new casualties, violence overall was down compared to last year.
"Both coalition and Iraqi security force casualties are down significantly from about May of '07," he said. "Iraqi civilian casualties has also been on a downward trend since December of '06."
"Would we like to reduce the casualties to nothing? Of course we would. Are there still going to be casualties in the days ahead? Most unfortunately there will be."