There are confusing reports about at least one explosion in the western Iraqi city of Ramadi.
Iraqi officials said 18 people, most of them children, had been killed in a blast near a football pitch.
Later a US spokesman told Reuters news agency that US forces had carried out a controlled explosion in Ramadi, also close to a football field.
The official said there were injuries, but no deaths. It was not clear if both reports referred to the same incident.
However, the chief American military spokesman in Iraq, Lt Colonel Christopher Garver, later said he thought there had been "two separate incidents" in Ramadi.
Iraqi Prime minister Nouri Maliki appeared to confirm the killings, calling them a "cowardly act" by "terrorist bands".
Iraqi police said most of the victims were aged 10 to 15 and had gathered to play football when a bomb went off.
Ramadi is the capital of Anbar province - the centre of Iraq's Sunni Arab insurgency.
On Saturday, a bomb near a Sunni mosque close to the city killed more than 50 people after the mosque's imam had made a speech criticising al-Qaeda, which correspondents say is entrenched in the area.
Violence continues to kills dozens of Iraqis every day
The BBC's Jane Peel in Baghdad says it would not be the first time children playing football have been caught up in the violence.
Last August at least 12 boys and young men died when a bomb exploded on a football pitch in the capital.
There has been a sharp rise in violence between Iraq's Sunni and Shia Muslim groups in the past year.
Iraqi and US forces have launched an operation aimed at stemming sectarian attacks in the Baghdad area.
The Ramadi attack was reported as other bombings killed at least 18 people across Iraq.
Also on Tuesday, four bombings in the Iraqi capital killed 12 people, including three US soldiers.
Two of the bombings took place in the Karrada commercial area.
Meanwhile, in the northern city of Mosul, a suicide bomber drove a lorry into a police station, killing six people and wounding 38 others.