Papua New Guinea's (PNG) Prime Minister has offered condolences after 16 women and children died in a tribal killing in the country's Hela Province.Prime Minister James Marape took to social media on Tuesday evening to offer his condolences. He wrote: "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." 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.""Before (becoming Prime Minister), I had someone else to report to, now I have no one else to report to but the innocent you kill.""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. Last week I responded to question on death penalty on the floor of parliament, it is already a law," he warned.While it is not yet clear what the exact circumstances were that led to the killing on Monday, Hela Governor Philip Undialu told the Australian Broadcasting Corporation on Wednesday that 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.