The victims of Monday's attack are all believed to be members of one of several rapidly expanding U.S.-backed volunteer security groups.
The groups, often known as "awakening councils" -- also referred to as Concerned Local Citizens by the U.S. military -- are mainly composed of Sunnis who seek to oust the al-Qaida in Iraq insurgents and are said to have some 70,000 fighters in their employ who are paid by the U.S. military to protect their neighborhoods.
In Monday's attack, a suicide bomber drove a minibus rigged with explosives into a checkpoint in Tarmiyah, 30 miles north of Baghdad, police and a member of the local council told the Associated Press. The explosion killed 12 people, said Adil al-Mishhadani, a member of the council.
In an audiotape released Saturday, Osama bin Laden criticized such groups for fighting with Americans against al-Qaida, saying "the most evil of the traitors are those who trade away their religion for the sake of their mortal life."
Leaders of the volunteer groups, credited with helping slash violence across the country by 60 percent since June, condemned bin Laden's latest message to his followers.
"We consider our fighting against al-Qaida to be a popular revolution against the devil," said Sheik Mohammed Saleh al-Dohan, head of one of the groups in southern Ramadi, a city in Anbar province where the movement was born, according to the AP.
Another contributing factor in the reduction in violence in the second half of the year was a cease-fire pact by the Mahdi Army, led by Shiite extremist Muqtada al-Sadr.
Despite an overall dip in violence, 2007 still proved deadly for U.S. soldiers, who suffered 899 reported fatalities. The worst month for U.S. deaths was May, just before the surge of 30,000 additional U.S. troops deployed to Baghdad and surrounding areas; 21 deaths have so far been reported in December, the second-lowest monthly total during the course of the war.
At least 3,902 members of the U.S. military have died since the beginning of the war, according to the AP.
Iraqi civilian deaths saw a similar peak and subsequent drop for 2007, with a total of 18,610 estimated killed for the year, according to AP surveys. Overall violence in Iraq has dropped an approximate 60 percent, American military commanders said."We realize that security in Iraq is very fragile and tenuous," U.S. Navy Rear Adm. Gregory Smith said, according to news agencies. Smith lauded the progress that has been made since the surge, but added that "there is no place in Iraq today that is safe from terrorism."