Ban "condemns in the strongest possible terms this brutal and senseless act of violence," the statement said, adding that he "extends his deepest condolences and sympathy to the families of the victims and sends his wishes for a speedy recovery to those who were injured."
At least 60 people were injured in the blast, which was caused by a truck bomb that exploded in the heart of the southern city of Kandahar.
The city has long been a stronghold of the Taliban insurgency, but the militant group denied responsibility for the attack.
It was the deadliest explosion in Afghanistan since a suicide car bomber killed more than 60 people, including two senior diplomats, in an attack on the Indian embassy in Kabul on July 7, 2008.
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