The death toll was expected to rise after the explosion at the Eidgah used-clothes market in Parachinar city, the capital of Kurram tribal district on the border with Afghanistan.
“At least 12 have died and around 50 have been injured,” said Amjad Ali Khan, the political administrator of Kurram.
Two senior police officials who requested anonymity confirmed the incident.
A doctor at the district headquarters hospital, where the wounded were taken, told AFP most of them were in critical condition and the death toll could increase.
No one has yet claimed responsibility for the attack.
Kurram is one of Pakistan’s seven semi-autonomous tribal districts which are governed according to local laws and customs.
They are the frontline of Pakistan’s battle against an Islamist insurgency that began in 2004 after the US-led invasion of Afghanistan forced Taliban and Al-Qaeda militants to flee across the border.