Gunmen opened fire on a passenger bus in the northern Pakistani district of Kohistan in an apparent sectarian attack on Tuesday, killing 18 people, police officials said.
"All the people on board were Shi'ites, and at the moment it looks like they were targeted by armed men from the local Sunni community," a senior police official told Reuters.
The bus was traveling from central Pakistan city of Rawalpindi, near the capital Islamabad, to the northern town of Gilgit.
Police officials said the bus came under attack in an area inhabited by two Sunni tribes about 165 km (102 miles) north of Islamabad.
The majority of Pakistanis are Sunni Muslims, with Shi'ites accounting for around 15 percent of a population of around 180 million.
Both communities largely live in peace with each other but militants from the two sides have killed thousands of people in tit-for-tat attacks since the beginning of Islamist militancy in the country in the 1980s.
Shi'ite Muslims are a minority sect of Islam, arising from a dispute over the successor to the Prophet Mohammad 1,400 years ago. Many extreme Sunni Muslims consider them apostates.