The girl appeared at press conference in Lashkar Gah, capital of the southern province of Helmand, where she told how her brother had forced her to wear the vest and ordered her to detonate herself at a police checkpoint.
"I was tired of my stepmother. My brother told me to wear the black vest, go to the police checkpoint and press the button," she told reporters.
"I went past a river and decided to drop the vest. My brother fled and police arrested me."
The Tolo TV news channel said the girl, named as Spozhmai, was unable to operate the button to detonate the explosives.
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The interior ministry said that her brother was a Taliban commander who coerced her into putting on the vest and walking towards the police post in Khanashin district in the southern heartlands of the insurgency.
Taliban militants, who have been fighting against the US-backed government since they were ousted from power in late 2001 after the 9/11 attacks, have been accused in the past of using young boys to plant roadside bombs and act as suicide bombers.