Gunmen shot dead a father, mother and at least three children early on Friday, while a husband and wife were killed in a separate attack, security and medical officials said.
The gunmen in military uniforms broke into a house in Mahmudiyah, 30 kilometers (18 miles) south of Baghdad, and shot dead the parents and three children, an interior ministry official said.
A medical source at the hospital in Mahmudiyah said it had received the bodies of eight members of a Shiite family -- a man, a woman and six children aged between four and 14 years old. All had been shot in the head.
Mahmudiyah is a mixed Sunni-Shiite town that was formerly a Sunni insurgent stronghold.
Gunmen also attacked a house in Saadiyah near Khanaqin, 150 kilometers (95 miles) northeast of Baghdad, killing Mohsen Hassan, an Iraqi communist party member, and his wife, Shahad Ahmed, a police lieutenant colonel and a Baquba hospital doctor said.
The Iraqi communist party was once of one the strongest in the region but was decimated by now-executed dictator Saddam Hussein.
The shootings come just two days after a wave of bomb and gun attacks killed 72 people and wounded more than 250 across Iraq, the country's deadliest day in almost 10 months.
Violence across Iraq has declined dramatically since the 2006-2007 peak but attacks remain common, especially in Baghdad. A total of 132 Iraqis were killed in May, official figures show.