The bus was transporting prisoners from a military base in the town of Taji to Baghdad when it was hit by roadside bombs, the sources said. Gunmen then opened fire.
The motive for the attack was not immediately clear.
Much of Iraq's bloodshed these days is linked to sectarian tensions which have been rising since Sunni militants seized large swathes of northern Iraq last month and declared an Islamic empire.
In June, 69 prisoners from an outlying town being transported to a jail in Baghdad were killed before reaching their destination.
The official account, given by the governor of Hilla, 92km south of Baghdad, hours after the killing, was that militants had attacked the convoy killing 10 prisoners and one policeman in the crossfire.
But a police captain, a second police officer and a senior local official from where the prisoners died in Hilla told Reuters no attack took place, and that police had executed the 69 men.
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