Police say they believe the girl's account, in which her brother asked the girl to wear a suicide vest and approach a police checkpoint to ask for a ride to a neighboring province.
The girl backed out of the plan at the last minute and her brother, identified as Zahir, fled with the vest, police said.
Karzai condemned the Taliban over the incident, but the group denied any involvement in the alleged plot.
Human rights groups say the insurgent group occasionally has dispatched children for suicide bombings, but girls have been rarely used for such attacks.
