Another five towns and cities were also targeted, which came a day after the US military confirmed a major troop reduction.
Troop numbers came down to 50,000 as Washington prepares to withdraw from its combat mission at the end of the month.
At their peak, during 2007's brutal Shiite-Sunni sectarian war that cost thousands of lives, US troops numbered around 170,000.
The spate of bloodshed over the past two months has triggered concern that Iraqi forces are not yet ready to handle security on their own.
It is said the police and army are singled out as targets of attack by armed groups in Iraq.