A suicide car bomb attack on a US Marine vehicle in the city of Falluja killed six American troops in one of the deadliest single assaults on US ground forces in Iraq, the US military said.
In Washington, US President George W. Bush vowed insurgents in Iraq would be defeated. "The way ahead is not going to be easy," Mr Bush told a White House news conference with Iraqi Prime Minister Ibrahim Jaafari.
"The enemy's goal is to drive us out of Iraq before the Iraqis have established a secure, democratic government. They will not succeed," said Mr Bush.
A US official said two Marines were killed and 13 wounded in the bomb attack late on Thursday in Falluja, west of Baghdad. Another four US troops, thought to be three Marines and a sailor, were presumed dead, suggesting their remains had so far not been identified. One of those confirmed dead and 11 of the wounded were women, a US military statement said.
Female Marines are used to search Iraqi women at checkpoints. The other Marine confirmed dead was killed by small arms fire immediately after the bomb attack. The deaths brought to at least 1,731 the number of US troops killed since the US-led invasion of Iraq in March 2003 to topple Saddam Hussein.
US officers say insurgents are developing more powerful "shaped" bombs that are capable of piercing if not destroying US armoured vehicles, greatly increasing the deadliness of their attacks.
Earlier this month, ten US troops were killed in two separate roadside bomb blasts near Ramadi.