Ten US marines were killed and 11 wounded by a roadside bomb outside the central Iraqi town of Falluja on Thursday, the US military has said.
More than 2,000 US troops have died in Iraq since the invasion
The soldiers from the 2nd Marine Division were conducting a foot patrol when a bomb exploded.
Their combat unit was taking part in a counter-insurgency operation in Anbar Province, west of the capital Baghdad.
The attack was one of the deadliest against US forces. It brings to 2,120 the number of US troops killed in Iraq.
US President George Bush was saddened by the deaths, spokesman Scott McClellan said.
"Our heart and prayers go out to the families, their loved ones paid the ultimate sacrifice for an important cause and we are forever grateful for their service and sacrifice," he added.
Seven of the wounded soldiers have returned to duty.
"Marines continue to conduct counter-insurgency operations throughout Falluja and surrounding areas to provide a secure environment for the national elections," the statement added.
The Pentagon says nearly 16,000 US troops have been wounded in combat since 2003.
US officers have repeatedly warned that the number of attacks carried out by insurgents in Iraq would increase in the run-up to elections on 15 December.
The attack is the deadliest against US forces in Iraq since 14 marines were killed by a roadside bomb in Haditha in August.
3 August 2005: 14 US Marines are killed in a roadside blast near Haditha, north-west of Baghdad
26 January 2005: 31 US Marines are killed when their helicopter comes down in bad weather in western Iraq
21 December 2004: 14 US troops are among at least 21 killed by a suicide bomber in a mess tent in Mosul
15 November 2003: 17 US soldiers are killed when their helicopters collide under heavy fire in Mosul
2 November 2003: At least 15 US troops are killed when their helicopter is shot down near Falluja
Unusually, the first announcement of the Falluja attack came out of Washington.
The BBC's correspondent in Baghdad, Jim Muir, says these are politically sensitive times when it comes to troop losses.
Falluja was the scene of a massive American led operation against the insurgents just over a year ago. It was meant to be a showcase for a campaign of clearing, controlling and rebuilding areas once considered hotbeds of the insurgency, our correspondent says.
The deaths also came just one day after President Bush's major speech outlining his strategy for victory in Iraq.