The ministry of interior said on Wednesday that six people had been killed and eight wounded in the explosion near Pul-e-Charkhi prison in Kabul.
This was down from the seven deaths reported earlier by Kabul police chief's spokesman Basir Mujahid who said at least 14 others were wounded.
MoI spokesman Najib Danish said three security force members and three civilians, including children, were killed in the attack.
A suicide bomber reportedly targeted a prison staff bus that was transporting employees to work.
The incident happened at about 7:30am local time after a vehicle belonging to Pul-e-Charkhi prison was targeted, according to the officials at the Ministry of Interior.
However, no group including the Taliban has claimed responsibility for the blast immediately.