Gunmen disguised in military fatigues hauled 18 Shias off buses on Tuesday and shot them dead in cold blood in Gilgit.
Terrorist group Jundallah claimed responsibility for the attack through its commander Ahmed Marwat.
The government of Gilgit-Baltistan announced a three-day of mourning over the loss of life and all educational institutes and public offices will remain closed in Gilgit for the next three days.
The attack took place in the northern district of Kohistan, which neighbours Swat valley, a former Taliban stronghold.
Police said the attackers flagged down buses, climbed on board asking passengers whether they were Shia or Sunni Muslim, then dragged out the Shias and shot them.
Another eight people were injured in the attack, including two women and three children. “The motive was sectarian. The gunmen were wearing army uniform,” Mohammad Ilyas, the police chief in Kohistan told AFP after the attack near the town of Harban, 130 miles (210 kilometres) north of the capital.
One bus and three minibuses were travelling from Rawalpindi, the city where the Pakistan Army is headquartered, to the northern town of Gilgit.
“They checked the identity of the passengers, got the Shias off the vehicles and shot them dead,” Ilyas said. “The dead were all male.”
Kohistan administration chief, Aqal Badshah, said 18 people were killed by eight attackers armed with Kalashnikovs and wearing military dress.
It was the fourth militant attack in the country’s north since Thursday, raising fears that violence linked to a Taliban insurgency is again on the rise following a decline in recent months.
Human rights groups have heavily criticised the Pakistani government for failing to crack down on sectarian violence between the country’s majority Sunni and minority Shia Muslim communities that has killed thousands.
Local MP Abdul Sattar Khan linked the ambush to the murder of two Sunni Muslims a few days ago in Gilgit.
“The people of the area had vowed they would take revenge,” Khan told AFP by telephone.
Some of Tuesday’s victims were from Gilgit, where the government ordered offices and schools to close as a safety precaution, and advised residents to stay indoors, local administration chief Tariq Arqam told AFP.
Residents said Gilgit was tense and roads deserted. Shops in most areas were closed and traffic very thin, they added.
KP Information Minister Mian Iftikhar Hussain blamed the attack on militants.
“The people behind this attack are terrorists. They want to trigger sectarian violence in the country. We don’t want to go into the details because we don’t want them to succeed in their nefarious designs,” he said.
“We will give cash compensation to the families of the victims. The bodies of the dead have been sent to Gilgit for burial,” he added.
Authorities had earlier insisted Islamist militants were not active in the area, although Kohistan borders Swat, where Pakistan in 2009 managed to put down a two-year Taliban insurgency.
Only a few days ago, a blast in Parachinar had killed 28 Shias and injured 36 close to an Imambargah.
Two similar bus attacks targeting Shias had taken place last year as well.
On September 20, gunmen had shot dead 26 Shia pilgrims travelling to Iran, causing the deadliest attack on the minority community for more than a year.
Just like Tuesday’s attack, gunmen had ordered pilgrims off their bus, lined them up and assassinated them in a volley of gunfire in Mastung, 50kms south of Quetta.
Later on October 3, terrorists had attacked a bus carrying Shias in the outskirts of Quetta, killing 13 people.