A mortar barrage on a US-run prison in Baghdad killed 22 prisoners and wounded more than 90 today, a US military spokesman claimed.
All of the casualties at the Abu Ghraib jail just west of Baghdad were among the 4,400 people detained there on security grounds, he said.
Insurgents also attacked a US military convoy in the northern city of Mosul and one American soldier died of his wounds. Four others were injured, an army spokesman said.
Elsewhere, tension eased in two flashpoint cities as a truce held in the Sunni bastion of Fallujah and US forces prepared to pull back from a forward base near Najaf, where rebel Shi'ite cleric Moqtada al-Sadr has taken refuge.
Witnesses said civilians who had fled battles between US Marines and Sunni insurgents trickled back to Fallujah on foot, though vehicles were turned back at checkpoints.
Some shops reopened and some Iraqi police returned to duty. There was no sign of US forces in central parts of the city.
Thousands of Iraqis had left Fallujah, 50 km (30 miles) west of Baghdad, to escape fierce fighting in which hundreds of civilians and dozens of Marines were killed this month.
The returnees were venturing back a day after the US military said it would not resume offensive operations in Fallujah on condition rebels gave up their heavy weapons.