A woman has been tortured and burned alive in Papua New Guinea after being accused of using sorcery to kill a young boy, local media report.

The woman, a mother aged 20 named as Kepari Leniata, was stripped, tied up and doused in petrol by the boy's relatives in Mount Hagen in the Western Highlands, said the National newspaper.

She was then thrown onto a fire in front of hundreds of people.

Police and firefighters were unable to intervene, the paper said.

The Post Courier newspaper said they had been outnumbered by the crowd and chased away. Both newspapers published graphic photos of the incident on their front pages.

Provincial police commander Supt Kaiglo Ambane told the National that police were treating the case as murder and would arrest those responsible.

In parts of the Pacific nation deaths and mysterious illnesses are sometimes blamed on suspected sorcerers. Several reports have emerged in recent years of accused people, usually women, being killed.

In 2009, after a string of such killings, the chairman of PNG's Constitutional Review and Law Reform Commission said defendants were using accusations of witchcraft as an excuse to kill people, and called for tougher legislation to tackle the issue.

Local Christian bishop David Piso told the National that sorcery-related killings were a growing problem, and urged the government "to come up with a law to stop such practice".

The US embassy in the capital, Port Moresby, condemned the killing as a "brutal murder", the AFP news agency reports, and evidence of "pervasive gender-based violence" in Papua New Guinea.

"There is no possible justification for this sort of violence. We hope that appropriate resources are devoted to identifying, prosecuting, and punishing those responsible for Ms Leniata's murder."