The execution was carried out on Friday in Mehraban Kalay area of Tirah valley in Khyber district.
The militants killed the tribesman in a market and ordered local people not to remove the body till evening, the Dawn reported on Saturday.
Mehraban Kalay and adjacent areas are under control of the outlawed Tehreek-i-Taliban Pakistan, but Lashkar-i-Islam also has some influence.
The two groups joined hands after Lashkar's chief Mangal Bagh invited all outlawed militant groups to come to Bara and fight against security forces.
The security forces have launched the Khyber-1 military operation against militants in the area on October 16. The operation has forced thousands of people to flee their homes.
In Bara, security forces and local political administration extended the deadline to today for Sipah and Malikdin Khel tribes to vacate their homes.
The deadline was extended after tribal elders sought more time to shift families to safe places.
The Fata Disaster Management Authority has said that 53,819 families comprising 364,960 people have so far moved out of Bara and Tirah valley.
There are 213,195 children and 77,319 women among them.
Each family had been provided with one meal, free transportation and emergency health care.
Children under the age of five have been vaccinated against polio. Also on Friday, 39 militants belonging to LI surrendered to security forces in Khyber.
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