The incident took place on Friday afternoon as scores of people had gathered in a mosque for Friday prayers in a remote village in the Mohmand Agency, north of Peshawar.
According to the political administration sources the explosion took place when people were returning home from the mosque after Friday prayers.
However, a senior tribal administration official told AFP news agency: “The Friday prayer was in progress at the mosque when a suicide bomber blew himself up in the main room.”
A faction of the Pakistani Taliban, Jamaat-ul-Ahrar, has reportedly carried out the attack which apparently targeted elders from a government-sponsored anti-Taliban militia.
The tribal areas of Pakistan have been subject to recent attacks both by the Pakistani Taliban and the loyalists of the Islamic State.
The violence sporadically continue in the area despite the Pakistani military began operations there in 2014 to reduce the strength of the Taliban.