At least seven people were killed and more than a dozen remain missing after gunmen ambushed a public bus and shot passengers in Papua New Guinea’s Enga province.

The shooting took place in Enga’s Lagaip district on Tuesday afternoon. More than 20 passengers were on the bus when men armed with high-powered weapons “set up an ambush and fired shots indiscriminately” into the vehicle, highland western end assistant police commissioner Joseph Tondop said in a statement on Wednesday.

Seven male passengers, including the driver, were killed. Local police said the bus driver turned the vehicle around after the attack and drove Wabag hospital, about 50 minutes away, but later died from the bullet injury.

One person remains in a critical condition in Wabag hospital and at least 17 are unaccounted for. Tondop said it was unclear whether the missing passengers are still alive or wounded, and may be hiding in the bushes or sheltering with neighbouring clans.

Early on Wednesday, police launched a search operation for the missing passengers, starting at the scene of the ambush, Tondop said.

Tondop said the attack was a “payback killing”. It is believed to be related to a grievance with one of the bus passengers and marks the latest outbreak in long-running violence between different tribal groups in Enga province.

Clans have fought each other in Papua New Guinea for centuries but the violence has become more deadly, largely due to an influx of more sophisticated weapons. Guns, machetes and hand grenades are regularly used in tribal fights.

Recent violence has claimed dozens of lives, destroyed homes, infrastructure and businesses in the province. In February, at least 50 people were killed in a massacre by rival tribes, in conflict that stemmed from long-running land disputes. It is complicated and governed by customs and beliefs that stretch back generations.