A security official said the twin blasts in the capital's Sunni enclave of Sheik Omar happened at a checkpoint manned by members of an Awakening Council, the term for mostly Sunni groups that have joined forces with the Americans against al Qaeda in Iraq.
Two civilian bystanders were among those killed in the blast, the official said.
In Baghdad's sprawling Shiite slum of Sadr City, a roadside bomb exploded next to a pickup truck carrying construction workers on their way to work, police said. Nine people were injured in the attack.
The officials spoke on condition of anonymity because he was not authorized to release information to the media.
Violence has dropped sharply in Baghdad since the Sunni revolt - led by Awakening Councils - against al Qaeda and the routing of Shiite militias in Baghdad and southern Iraq last spring.
The decline in violence across Iraq has led American military commanders to announce that they willthis month - at least two months earlier than originally planned.
Military officials said Wednesday two brigades from the 101st Airborne Division would leave Iraq this month and only one will be replaced. A brigade is roughly 3,500 soldiers. Initially the 3rd Brigade, 101st Division, was scheduled to leave this month, and the 2nd Brigade, 101st Division, was to leave by February.
But there has been a marked uptick in violence this week, with a string of daily bombings in the Iraqi capital that have killed more than 30 people and wounded around 80 others.
U.S. officials say attacks in Baghdad are averaging about four a day - down nearly 90 percent from levels in late 2006, when Shiite-Sunni fighting was at its high point and just before the U.S. troop surge that helped bring down violence in the capital. But U.S. commanders warn the security gains are reversible.
Also on Thursday, in the ethnically mixed city of Kirkuk, 180 miles north of Baghdad, police found the bullet-riddled body of a man believed to be in his late 40s.
Police said the man had bullet wounds in his head and chest and that his body bore signs of torture.