Local officials said at least 24 people had been killed in Hela province in the west of the country.
"Today is one of the saddest days of my life", Marape, who became the South Pacific country's prime minister in May, said in a statement.
Mr Marape, who became leader in May, blamed a lack of police on the ground as a major problem in a region that has struggled with violence for years.
He said while the motive behind the killings was not known, he believed it could be in retaliation for an earlier incident.
The disputes are often over land allocation and the distribution of resource wealth.
Media captionPort Moresby: Doubtlessly the most deadly metropolis on this planet to be a girl?How did the killings unfold?
How did the killings unfold?
Particulars are silent emerging from the faraway central home, however the violence appears to be like to were a string of attacks and counter-attacks over several days.
Local media report at least two incidents in small villages in the Tari-Pori district. The provincial police commander, Teddy Augwi, said the attackers used high-powered rifles.
"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.
In a Facebook post on Tuesday, Pills Pimua Kolo, a worker from the province's Department of Health said the attack took place on Monday morning around 6 a.m. local time. "Some chopped into pieces and hard to recognize (sic) their body parts".
How did the authorities respond?
Prime Minister Marape said he was "coming for" the killers.
"How can a province of 400,000 of us feature with policing regulations and expose with below 60 policemen, and espresso operational protection power and police that does no higher than band abet maintenance", he added.
How general is tribal violence in PNG?
Brutal tribal violence is a common and ongoing occurrence in Papua New Guinea's Highlands and officials have always been trying to tackle it, citing its effect on the nation's development.
In 2018, an quake devastated some of the remote regions.
Tribal clashes are no longer irregular with rivalries incessantly led to by rape or theft, or disputes over tribal boundaries.
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