"Three soldiers traveling in a 1st Armored Division convoy were killed during a small arms ambush" near the southern town of Diwaniyah around 7:00 pm (1500 GMT), the military said in a statement.
Clashes broke out after the convoy was attacked on Saturday night, in which witnesses said at least seven Iraqis were killed.
Another army statement said "a soldier assigned to the 1st Marine Expeditionary Force was killed yesterday as a result of enemy action in the al-Anbar Province," west of the Iraqi capital.
Al-Anbar includes the town of Falluja where US forces have launched an ambush on the town's resistance fighters and its residents. More than 600 people have been killed there in just over a week.
The province has proved a hotspot for resistance to the US-led occupation.
At least 498 US soldiers have been killed in action since the US-led war in Iraq began 13 months ago.