At least 64 people were massacred in Papua New Guinea's remote highlands, in an escalation of long-running violence between local warring tribes, police said Monday.

The victims were believed to be tribal fighters who were ambushed by a rival group in the early hours of Sunday.

The incident occurred near the town of Wabag, about 600 kilometers (370 miles) northwest of the capital Port Moresby.

The remote and rugged area has for years been the scene of tit-for-tat mass killings between rival Sikin, Ambulin, Kaekin and other tribesmen.

Graphic police images from the scene showed stripped and bloodied bodies lying by the side of the road and piled up on the back of a flatbed truck.

Some men had limbs hacked and were left naked by the road with beer bottles or cans placed on their chests.

Police on Monday said gunfights were ongoing in nearby valleys and bodies were still being recovered from bushland near the road.

"We believe there are still some bodies ... out there in the bush," Assistant Commissioner of Police Samson Kua said.

Clans have fought each other in Papua New Guinea's highlands for centuries, but an influx of mercenaries and automatic weapons has made clashes more deadly and escalated the cycle of violence.

Kua said the gunmen had used a veritable armory, including SLR, AK-47, M4, AR15 and M16 rifles, as well as pump-action shotguns and homemade firearms.

Mass killings

The province's acting police commander Patrick Peka said many of the dead were believed to be mercenaries – men who roam the countryside offering to help tribes settle scores with their rivals.

"The police and government cannot do much when leaders and educated elites supply arms, ammunitions and engage the services of gunmen from other parts of the province," Peka said.

Papua New Guinea's government has tried suppression, mediation, gun amnesties and a range of other strategies to control the violence, with little success.

The military had deployed about 100 troops to the area, but their impact has been limited and the security services remain outnumbered and outgunned.

The killings often take place in remote communities, with attackers launching raids or ambushes in revenge for previous attacks.

'Very disturbing'

Civilians, including pregnant women and children, have been targeted in the past.

The murders are often extremely violent, with victims hacked with machetes, burned, mutilated or tortured.

Police privately complain that they do not have the resources to do the job, with officers so badly paid that some of the weapons that end up in the hands of the attackers have come from the police force.

Opponents of Prime Minister James Marape's government on Monday called for more police to be deployed and for the force's commissioner to resign.

Papua New Guinea's population has more than doubled since 1980, placing increasing strain on land and resources and deepening tribal rivalries.

Anthony Albanese, the prime minister of neighboring Australia, on Monday described the incident as "very disturbing."

"We are providing considerable support, particularly for training police officers and for security in Papua New Guinea," he told public broadcaster ABC.

"We remain available to provide whatever support we can."