On a highway about nine miles northwest of Baghdad, armed men stopped a minivan and assassinated all 13 occupants, including an elderly woman and two boys, accusing them of opposing al-Qaeda in Iraq, police and witnesses said.
North of Baghdad, a suicide car bomber attacked a house in the Khazraj district belonging to an Iraqi army major, killing five soldiers and wounding 10 others, police said.
In a separate incident, one person was killed and seven others were injured in a car bomb explosion in Mahmudiya, south of Baghdad, police said.
In the northern city of Tal Afar, five people, including a six-year-old boy, were killed by a roadside bomb, Najim Abdullah, a local official, said.
US soldiers killed
US forces came under attack in Tarmiyah, about 50km north of Baghdad, on Monday after a suicide bomber apparently tried to break through barriers around a joint base with Iraqi forces.
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"Suicide attackers, driving booby-trapped cars, have stormed the base when a large number of US troops were gathering there," Ali Salem, an Iraqi journalist, told Al Jazeera.
Two US soldiers were killed and 17 more wounded in the attack, the US military said.
Elsewhere, three US soldiers were killed and two were wounded in a roadside bombing southwest of Baghdad, the US military said.
The military also announced the deaths of three marines and one soldier since Saturday while conducting combat operations in the western province of Anbar.
Altogether, nine US service members have been reported killed since the beginning of the weekend, six of them on Monday.
Also on Monday, two suicide car bombers blew themselves up in Ramadi in Anbar province, killing at least 11 people and wounding four others, police and witnesses said.
The first explosion hit the blast walls outside the house of Abdul Setar Abu Risha, a tribal leader who has led a government-backed campaign to fight al-Qaeda-linked fighters in Anbar.
The second suicide bomber crashed his vehicle into the house before blowing himself up.
Five police officers and six civilians were killed in the blasts, witnesses said.
Tens of thousands of US and Iraqi troops have been deployed in Baghdad as part of Operation Fardh al-Qanoon - Operation Imposing Law - which officially started last week.
"As displaced families return home peacefully, and hopes are raised by 'Operation Fardh al-Qanoon', criminal terrorists are not happy to see life returing to normal in Baghdad," Nuri al-Maliki, the Iraqi prime minister, said.
"Our dear people in Baghdad and across Iraq's provinces have chosen to go ahead with the political process despite sacrifices.
"They are determined to stand by our armed forces to encourage them to drive out remnants of the Saddamists and Takfiris and all outlaws."