Iraqi security officials say gunmen have killed 13 people west of Baghdad, while in the northern city Kirkuk, a car bomb exploded in a market, killing at least five.
The latest violence comes as Iraqis gear up for general elections planned for January. While overall security has improved, the attacks are a reminder of how precarious the situation is.
Iraqi security spokesman General Qassem Mohammed Atta addressed the execution-style killings in the mostly-Sunni area west of Baghdad.
Atta described them as "cowardly, terrorist" acts. He said a preliminary investigation indicated that the attackers had disguised themselves as members of the Iraqi army.
No one has claimed responsibility for the killings or the bombing in Kirkuk.
Tensions are high in the oil-rich northern city, a point of contention that led to delays in Iraq's recently-approved electoral law.
Most Kurds in Kirkuk want to be part of the Kurdistan Regional Government, while Arab and Turkomen residents want to be under the central control of Baghdad. Lawmakers reached a provisional compromise that could leave open the future of the city.
Passage of the electoral law was hailed by U.S. President Barack Obama as an important milestone. American plans to withdraw troops from Iraq are based in part on having the elections go forward.
But Iraq's Vice President Tareq al-Hashemi, a member of Iraq's Sunni minority, has raised objections to the current law. He said Monday he wants the number of parliamentary seats set aside for minorities to be raised, threatening to use his veto power over the law if the number is not changed.