Thousands of US troops are said to be based in Balad
US troops have killed 11 Iraqis who ambushed their convoy north of Baghdad, a US military spokesman has said.
The Iraqis attacked the convoy near the town of Balad using rocket-propelled grenades and small arms fire, the spokesman said.
Only a few hours earlier, 18 US soldiers were injured when mortar bombs were fired at one of the largest US military bases in Iraq, also near Balad.
The US is engaged in a big military operation in the area to root out supporters of ousted President Saddam Hussein, who have been staging increasingly bold attacks on American forces.
On Thursday night, a sniper shot dead a US soldier guarding the Iraqi National Museum in Baghdad.
No US troops were injured in the attempted ambush, the US military said.
All of the Iraqi attackers were killed, a statement said.
At least 19 American soldiers have been killed in combat since US President George W Bush declared major hostilities over on 1 May.
The latest casualties come as Americans celebrate their Independence Day with polls showing public scepticism about the pace of post-war reconstruction.
In the attack which injured 18 Americans, a US military spokesman told the BBC that three or four mortar rounds were seen exploding in the area around their logistical support base.
One of the mortar rounds exploded within the base, described as one of the Americans' largest in Iraq. Thousands of troops are understood to be stationed there.
President Bush is to stress the difficulties ahead in a message to mark Independence Day.
"Drawing on the courage of our Founding Fathers and the resolve of our citizens, we willingly embrace the challenges before us," he will say.
On Wednesday Mr Bush said the US would deal harshly with those responsible for the escalating attacks against its troops in Iraq.
"Bring them on," he said.
But he has faced criticism from opposition Democrats who have accused him of "macho rhetoric" that will only invite more attacks on US troops.
In the past week alone, fatal incidents have included:
- The killing of a marine on 2 July during a mine-clearing operations in Karbala.
- The death of a US soldier from wounds received when a convoy was hit by explosives in Baghdad on 1 July
- On 28 June the remains of two soldiers missing since 25 June were recovered 32 kilometres (20 miles) outside Baghdad
British soldiers in the south have also come under attack. Six were killed a single incident last month.
HAVE YOUR SAY
I do not think finding Saddam is important. This money could be used to embark upon infrastructural projects in the whole of the country
The US military authorities in Iraq have offered a $25m reward for information leading to the capture of Saddam Hussein.
His whereabouts have been the source of much speculation since Baghdad fell to US-led forces on 9 April.
The BBC's Richard Galpin in Baghdad says the capture or death of the deposed Iraqi leader and his sons would have much more than just symbolic value, as his supporters are believed to be responsible for the recent violence.