One report said a six-year-old girl was among the dead.
The checkpoint was jointly manned by Iraqi and US security forces at the time of the attack.
In other incidents across Iraq on Friday, a car bomb killed an Iraqi soldier in west Baghdad when it exploded as a military patrol drove by, an Interior Ministry official said.
And a Sunni Muslim imam was killed when a car bomb exploded outside his mosque in Samarra, scene of an attack last month on a major Shia shrine.
Security officials, eager to play down talk of sectarian violence, denied the mosque was targeted, saying the bomb went off as a police patrol drove by.
A woman and child were hurt in the blast, they added.
In east Baghdad, a roadside bomb exploded next to a US M1Abrams tank, setting it on fire, but the US military said none of its crew was hurt.
Four unidentified bodies of men who had been shot dead were also recovered on Friday in the capital, an Interior Ministry official said.
The latest attacks come a day after 11 people died in a spate of attacks across Iraq.
Over the past two weeks - since the bombing of the Samarra shrine - violence across the country has become increasingly sectarian leaving hundreds killed and injured.