A suicide bomber detonated his explosives as US soldiers were handing out toys to children at a playground northeast of Baghdad, killing at least three children and three of the troopers, US and Iraqi authorities said.
Seven children were wounded in the Sunday attack in Baqubah, where US soldiers wrested control from al-Qaeda in Iraq last summer.
The attack, along with a series of other blasts in the capital and to the north, underlined the uncertainty of security in Iraq even as the US military said overall violence is down 55 percent since a troop buildup began this year.
Police said the attack occurred as US soldiers were handing out toys, sports equipment and other treats at a playground near Baqubah, 56km northeast of Baghdad.
Few details were available, but the US military said it was a "suicide vest attack" and that three US soldiers were killed.
Rasoul Issam, 16, said he and his friends were playing soccer when the US soldiers called to them from their vehicles to come get gifts.
"We ran toward them and I caught a ball when suddenly an explosion took place about 20m from us," Issam said from his hospital bed in Baqubah.
Mohammed Sabah, 11, was hit by shrapnel in his hand and chest.
"The soldiers gave me pens and I thanked them. After this the explosion took place and I was hit by shrapnel," he said. "The second thing I remember is being in the hospital."
The deaths raised to at least 3,870 members of the US military who have died since the Iraq war started in March 2003, according to an Associated Press count.
The military cast blame on al-Qaeda in Iraq (AQI).
"This is another example of how AQI cares nothing about the Iraqi people. They will kill children to meet their goals," said Major Peggy Kageleiry, a spokeswoman for US forces in northern Iraq.
Iraqi children frequently converge on US troops, who usually carry soccer balls and stuffed animals crammed in their armored vehicles as they seek to garner good will.
In July 2005, a suicide car bomber sped up to US soldiers distributing candy to children July 2005 and detonated his explosives, killing up to 27 people, including a dozen children and a US soldier.
That occurred nine months after 35 Iraqi children were killed in a string of bombs that exploded as US troops were handing out candy at a government-sponsored celebration to inaugurate a sewage plant in west Baghdad.
Rocket and mortar barrages also hit several US bases in Baghdad overnight.
Rear Admiral Gregory Smith, a US military spokesman, said the attacks caused some casualties but no deaths.
"The fight we're up against has not gone away. Today's mortar and rocket attacks demonstrate that the enemy has the capacity to wage violence," Smith said. "We're working our way through those attacks and the level of damage."
In all, at least 29 people were killed on Sunday, including the three soldiers.
The deadliest attack was a parked car bomb targeting a convoy carrying Salman al-Mukhtar, an adviser to the Iraqi finance minister. Al-Mukhtar escaped injury, but the blast in the predominantly Shiite district of Karradah in central Baghdad killed at least 10 people and wounded 21, including two of the official's bodyguards, according to police and hospital officials.
The chief editor of an independent daily newspaper, al-Bayan al-Jadid, Sattar Jabbar, was in the car with the minister's adviser when the explosion occurred but also was not hurt, Jabbar's brother, Abdul-Wahhab, said.