The bomb went off in a market in Ghormach district, Faryab province. Although donkey bombs are rare in Afghanistan, they are occasionally used by insurgents as a way of getting close to targets.
A statement from the interior ministry in Kabul said that one policeman was killed and 17 civilians wounded in the incident.
A spokesman for the Faryab police chief, Sayed Massoud Yaqubi, could not confirm that a policeman had been killed.
But he added: "This morning a bag of explosives strapped around a donkey detonated, wounding 18 people."
Afghan government forces are fighting a bloody war against a decade-long, Taliban-led insurgency with the help of around 140,000 international troops stationed in Afghanistan.
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