At least 18 people have been killed and 30 injured in an outbreak of sectarian violence in north-western Pakistan, officials said on Saturday.

At least 58 people have died in the clashes since Thursday.

Unidentified gunmen opened fire on passenger vehicles on Thursday, killing over 40 in the Kurrram district, where armed Shia and Sunni Muslims have engaged in sectarian rivalry for decades over a land dispute near the Afghanistan border.

Officials said most of the dead were Shiites, sparking retaliatory attacks by armed groups, with markets and schools remaining shut in a curfew-like situation.

More violence broke out on Friday night, local time, when armed men attacked a village in the district, Khyber Pakhtunkhwa province chief secretary Nadeem Aslam Chaudhry said.

"They set on fire petrol stations and damaged properties as part of revenge," he said, adding police officials would be visiting the area and engaging elders on both sides to restore peace in the area.

Kurram-based journalist Rehan Muhammad said he had to flee his home after "gunfire suddenly erupted" after sunset on Friday.

"I realised it was an attack in retaliation for [Thursday's] incident and immediately grabbed my children, despite the bitter cold, and told my family to flee our home towards the mountains on foot," he said.

"At dawn, someone shouted that the attackers had left. When I returned, nothing was left. All that remained of my house was a pile of charred debris."

AP reported on Saturday that 33 people were killed in the latest violence, citing an unnamed official.

A police official requesting anonymity told Reuters that the death toll from the fresh violence could have been higher had residents of the village that was attacked not already evacuated their homes in anticipation of more violence.

He said the residents of Bagan village, a mostly Sunni area, had already left their homes and shifted to safe places in Lower Kurram.

Reuters