Sectarian gunmen disguised in military fatigues hauled 18 Shiite Muslim men off buses on Tuesday and shot them dead in cold blood in a usually quiet region of northern Pakistan.
Authorities blamed the assault on Islamist militant groups, without naming a specific organisation. The attack took place in the northern district of Kohistan, which neighbours the Swat valley, a former Taliban stronghold.
Police said the attackers flagged down buses, climbed on board asking passengers whether they were Shiite or Sunni Muslim, then dragged out the Shiites and shot them.
Another eight people were wounded 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 said 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 Shiites 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 Pakistan's northwestern Khyber Pakhtunkhwa province 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 Shiite 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 said 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 said.
Residents said Gilgit was tense and roads deserted. Shops in most areas were closed and traffic very thin, they added.
Provincial information minister Mian Iftikhar Hussain blamed the attack on militants.
"The people behind this attack are terrorists. They wanted 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.