Sydney, Jul 10 (IANS): The death toll of a tribal massacre in Papua New Guinea's Hela Province (PNG) has increased to 23, media reported on Wednesday.

Local television station EMTV reported that in addition to the 16 people violently murdered on Monday, seven people including four men and three women were killed in related incidents on Sunday, reported Xinhua news agency.

While it is not yet clear what the exact circumstances were that led to the massacre, Hela Governor Philip Undialu told the Australian Broadcasting Corporation on Wednesday, the motive behind the killings could be in retaliation for an earlier incident in another village.

"We've never heard of tribal fights happening in this area, it was a spillover of fighting happening in another area, that no-one expected," he said.

PNG Prime Minister James Marape took to social media on Tuesday evening to offer his condolences to a nation in mourning.

"Today is one of the saddest days of my life, many children and mothers innocently murdered in Munima and Karida villages of my electorate by Haguai, Liwi and OKiru gunmen," he said.

A native of the Highland region's Hela Province, Marape vowed to hold those responsible to account.

"In memory of the innocents who continue to die at the hands of gun-toting criminals, your time is up."

"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.

"Last week I responded to question on the death penalty on the floor of parliament, it is already a law," he warned.