Nearly all the residents of Koge watched as Julianna Gene and Kopaku Konia were dragged from their homes, to be hung from trees and tortured for several hours with bush knives. No one came forward to help. In the eyes of the villagers, the women were witches. They deserved to die.

"They used their powers to bewitch a man to death," said Kingsley Sinemane, a community leader. "We had to get rid of them, as they could have killed others. We had to protect our village."

The finger of suspicion fell on the women after a local man died in a car accident. The only sign now of the horror that unfolded in this remote Papua New Guinea village is a black, charred clearing where some dozen homes once stood. Fear of the supernatural and the stigma of being branded a witch is so great that around 30 of the victims' relatives were chased out of the village. Upturned shoes and a few bundles of clothes are all that remain of their former lives. Most of them had nowhere to flee to, as word had spread they were related to so-called witches. They are now forced to live in slums in the nearest town.

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A shocking increase in witch-hunt deaths in Papua New Guinea has prompted the government to launch a parliamentary commission of inquiry with a view to toughening the law. Joe Mek Teine, the chairman of the nation's law reform commission, has publicly declared that sorcery killings are "getting out of hand". Most witch hunts happen in the Highlands, the remote mountainous interior wracked by centuries of tribal wars and blood feuds. Contact with the outside world was only established in the 1930s, when some of the many ethnic groups were still living stone-age existences. Although there are no official statistics on sorcery killings, more than 50 were reported to the police in just two Highland provinces last year.

The homicide squad at Kundiawa police station in Simbu, a steep, rugged province believed to be the epicentre of the witch hunts, is struggling to cope with the surge in murders. Detective Inspector Blacky Koglame estimates there are up to 20 killings a month in this area alone, most of which are not reported. Witchcraft is a secretive and taboo subject, so people are often too scared to come forward.

A worrying new development is that the crime, which has historically been a rural phenomenon, is now spreading to urban areas as families are driven out of villages by poverty and tribal fighting, and into towns and cities. Mount Hagen, the largest city in the Highlands, has recently been rocked by a wave of witch killings, and there have even been cases reported in the capital, Port Moresby.

Belief in black magic is so ingrained that the government legally recognises sorcery, under the 1976 Sorcery Act. It permits white magic (healing or fertility rites for example) but the so-called black arts are punishable by up to two years in jail. This has resulted in murderers alleging the use of black magic as provocation and securing reduced sentences.

Branding someone a witch is a crime, but Detective Koglame estimates that fewer than 1 per cent of cases end up in court. Even when witnesses do come forward, he admits the police simply do not have the resources to investigate.

"Sometimes we have to borrow pens and paper from complainants in order to file our reports, and we can't always get to crime scenes as we don't have enough petrol money for the police cars," Det Koglame said. "And anyway, arresting people is very hard. Everyone in the community is usually involved, so you can't just go in looking for suspects, as you'd have to arrest the whole village, and that's impossible."

Those accused of sorcery are sometimes tried in local kangaroo courts, with tribesmen and village councils handing out death sentences. The killings are mostly committed by groups of men who first torture so-called witches to make them "confess" to their crimes and force them to name other "witches". Some villages even have groups of vigilante killers who will strike as soon as a suspect is named.

In one area deep in the Highlands a team of eight "witch hunters" claim to have tortured and killed 18 people between them. "Mostly a witch hunter just collects information from any village where there is a problem, and then we go in and collect those people who are suspected of being witches," said the leader of the group, a man with a reputation as a violent local gangster who refused to be named. "It is part of my culture, my tradition, it's my belief. I see myself as a guardian angel. We feel that we kill on good grounds and we're working for the good of the people in the village," he said. Witch hunts nearly always occur after a death or an illness of a community member. "Natural causes for death or illness are just not accepted," said Pastor Jack Urame, a researcher at the Melanesian Institute and one of the country's leading experts on sorcery killings. "So whenever someone dies in a village, a person must blamed," he said. According to Mr Urame the victims are typically older women or women on their own, who have no extended family to defend them. Witch hunts can also be used as a pretext to settle scores or land disputes he said.

Umame Gamano survived a witch hunt after she was accused of causing her husband's death. One of her daughters helped her to escape a baying mob armed with bush knives after she had been hauled before a community gathering at which villagers urged her to confess to being a witch. Ms Umame fled the village, leaving her children behind and has not seen them since the attack that happened over a year ago. Her attackers were her own relatives and have threatened to kill her children if they visit her.

"I don't know why they think I'm a witch and I don't understand why they think I killed my husband," she said. "I loved him so much."

Unreported World: Bush Knives and Black Magic, Channel 4, tonight, 7.35pm