Police and soldiers have been sent to Papua New Guinea's highlands to make arrests and provide security after more than 20 people, mostly women and children, were slain in recent tribal violence over a feud that has lasted for years.

In the latest attack, 16 women and children were killed this week by assailants armed with rifles in the village of Karida in Hela province, said Papua New Guinea Acting Police Commissioner Francis Tokura.

Because two of the victims were pregnant, police put the death toll at 18. Officials differ on whether the attack happened on Sunday or Monday.

Authorities say the slaughter was probably retaliation for an ambush near Peta village on Saturday that left six or seven people dead.

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Tokura said 20 police officers and 10 soldiers were sent to Hela to "stop any further violence and capture the killers."

"This is a tragedy. It is an unbelievable, unthinkable and atrocious criminal act that must be dealt with swiftly and severely," Tokura said in a statement.

"We will use all available resources to bring the killers to justice," he added.

Prime Minister James Marape said many of the victims lived in his electorate. "Today is one of the saddest days of my life," he said on social media.

Marape blamed a police shortage in Hela for the lawlessness.

"How can a province of 400,000 people function with policing law and order with under 60 policemen ...?" he wrote.

"To all who have guns and kill and hide behind the mask of community, learn from what I will do to criminals who killed innocent people, I am not afraid to use strongest measures in law on you," he said, referring to the death penalty.

Tribal violence is common in Papua New Guinea's interior, where villagers avenge relatives in retaliation known as payback.

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Hela official Rex Humbi said the latest violence was part of a tribal war that had been waged for more than 15 years.

"The people that got killed were innocent women and children," he said. "The police do go and investigate. They have the authority, of course, but don't have the firepower" to bring the killers to justice.

The killers were possessed by Satan and "feed on the soul, or the spirits, of the innocents that have been lost," Humbi said.

- AP