Gunmen opened fire on vehicles carrying mainly Shiite Muslims on a key highway in Pakistan's restive northwest on Thursday, killing at least 38 people and wounding 20 others, police said. 

Pakistan's interior minister referred to the attack as an act of terrorism, while Prime Minister Shehbaz Sharif said in a statement that "the enemies of peace in the country have attacked a convoy of innocent citizens, an act that amounts to sheer brutality." 

A local government official told the AFP news agency that police officers were among the dead; the convoy was traveling with a police escort.

Witnesses said that attackers had fired indiscriminately at a convoy of vehicles from either side of the highway over a period of several minutes.

Crowds gathered outside the hospital in the nearby city of Parachinar awaiting the wounded.

Sectarian tensions high in border region

The attack happened in the district of Kurram in Khyber Pakhtunkhwa province, where sectarian clashes between Sunni and Shiite Muslims have killed dozens of people in recent months.

Kurram is one of comparatively few areas in Sunni-majority Pakistan where Shiite Muslims make up the bulk of inhabitants.

It's also close to the border to Afghanistan, now governed by the fundamentalist Sunni Taliban, and an area of operation for anti-Shia militant groups like the Tehrik-e-Taliban Pakistan (TTP) and an "Islamic State" offshoot.

Around 50 people from either religious community in Kurram were killed in clashes in July connected to a land dispute, which led to an uptick in tensions and subsequent violence.

Thursday's shooting took place on a highway that had been closed for weeks following another attack on passenger vehicles last month; it was only reopened for those traveling with police escorts. 

Local police official Azmat Ali told the Associated Press (AP) that several vehicles were traveling in a convoy from the nearby city of Parachinar to Peshawar, the capital of Khyber Pakhtunkhwa.

No group immediately claimed responsibility.

On Wednesday this week, a suicide attack in the same province at a security checkpoint killed 12 soldiers and wounded several others.

msh/kb (AFP, AP, dpa)