A car bomb has killed five people between the Iraqi cities of Najaf and Karbala, sites of twin car bombings a week ago, the chief of police in Najaf said.
Ghalib al-Jazairi said those killed were civilians but the bomb in the town of Khan al-Nus appeared to have been aimed at a US military convoy. A witness confirmed that, saying a Landcruiser vehicle blew up as the military convoy passed.
Khan al-Nus lies about 30km north of Najaf.
Sunday's bombings in Najaf and Karbala killed more than 60 people and wounded nearly 200 in what was seen as an attempt to sow sectarian conflict between Shia and Sunni Iraqis.
Attacks on US forces are relatively rare in an area where Iraq's Shia majority predominates.
The area around Karbala was until a few days ago controlled by Polish forces.
Warsaw is withdrawing its troops from Iraq and US forces are due to take over in the city, which is 110km south of Baghdad.