One US soldier was killed and another seriously wounded when their jeep came under attack in Baghdad.
The Humvee was attacked as it drove under an overpass
A grenade was thrown at the 1st Armoured Division Humvee as it drove under an overpass in east Baghdad.
Nearly 550 US service personnel have been killed since Iraq was invaded by coalition forces last March.
The attacks came as up to 100 people were killed in blasts targeting Iraqi Shias celebrating a holy day in Baghdad and the city of Karbala.
The BBC's Paul Wood, in Karbala, said that following the attacks the atmosphere in the city turned extremely tense as shock turned to anger, particularly against anyone regarded as outsiders.
He said an Italian photographer and an Iranian had been attacked.
US military officials said the Baghdad grenade attack destroyed the Humvee in
which the soldiers were driving.
The wounded soldier was evacuated to a medical facility where he was in a serious condition.