The man, identified only as Nooruddin, was blindfolded and taken to an area near a cemetery in Parachinar, in the Kurram tribal region, where about 300 men threw stones at him until he died, The Express Tribune reported Wednesday.
An official of the local government and three tribesmen, who spoke on condition of anonymity, said the man, a government employee, had started the relationship while working in Parachinar. After he was transferred to another region, he continued the relationship.
Nooruddin returned to Parachinar Monday night planning to elope with the woman, but her family discovered his plan and locked him a room.
The local council of elders ruled according to their tribal code both the man and the woman should be killed. The Express Tribune said it had not determined whether the girl had been killed.
The local government has refused to confirm the incident.