DERA ISMAIL KHAN, Pakistan - Pakistani authorities on Wednesday said they were investigating the murder of six barbers in a tribal region that has long been a hotbed for militants looking to impose their version of strict Islamic law, which includes mandatory beards for men.

The six barbers were shot dead late on Monday night outside their residence in the North Waziristan district, and the killings have sparked fear amongst locals.

"It is not yet known what the motives of this incident are, we are investigating," district police chief Rokhan Zeb Khan told Reuters, confirming that the six barbers had been shot in the head.

The six, who ran different outlets offering grooming services, were not local residents and were living together close to their shops in the town of Mir Ali, where they were targeted by gunmen.

Khan said the barbers were from neighbouring province of Punjab. Previously outsiders have been attacked on allegations of being spies for Pakistan's security and spy agencies.

The attack comes amidst an upsurge in attacks by militant groups, particularly by the Tehreek-e-Taliban Pakistan (TTP). The TTP, however, denied it was responsible for the killings in a statement released to journalists by its spokesman.

No group has claimed responsibility.

"After this incident, I feel very scared, I don't know whether to go to work or not," a barber shop owner in another town of North Waziristan, Miranshah, told Reuters by phone, asking for his name not to be published for security reasons.

"We are even more afraid because we (barbers) are non-locals, and we live here in rooms or shops, so we have no personal security," he added.

Pakistani political leaders - particularly those based in the country's northwest - have already expressed concerns about the security situation ahead of national elections next month, questioning how proper elections could be held amidst an environment of fear.

(Writing by Gibran Peshimam; Editing by Alex Richardson)