U.N. High Commissioner for Human Rights Navi Pillay called on authorities in Papua New Guinea to investigate such crimes and bring their perpetrators to justice.
A 1971 law defining sorcery as a crime in the South Pacific nation should be repealed, Pillay's spokeswoman said.
"We are deeply disturbed by reports of the torture and killing of a 20-year-old woman accused of sorcery in Mount Hagen, Papua New Guinea, on February 6," U.N. human rights spokeswoman Cecile Pouilly told a news briefing in Geneva.
Kepari Leniata was burnt alive in front of a crowd by relatives of the 6-year-old boy whom she was accused of using sorcery to kill, she said. Attempts by law enforcement officials to intervene failed.
"We note with great concern that this case adds to the growing pattern of vigilante attacks and killings of persons accused of sorcery in Papua New Guinea," Pouilly added.
The U.N. human rights office was able to document a case of five people, three of them women, who were tortured for 20 days and killed last November after being accused of using sorcery to kill others in Jiwaka province, she said.
"We think it is clearly under-reported, because many of these cases happen in rural areas and go unreported. It is clearly deeply rooted," she told reporters.
Rashida Manjoo, U.N. special rapporteur on violence against women, met victims of sorcery during an investigative mission to Papua New Guinea last March. Widows or other family with no family to protect them are particularly vulnerable to such attacks, she said in a report to the U.N. Human Rights Council.
"I was shocked to witness the brutality of the assaults perpetrated against suspected sorcerers, which in many cases include torture, rape, mutilations and murder. According to many interviewees, sorcery accusations are commonly used to deprive women of their land and/or their property," she wrote.
The country's Constitutional Law Reform Commission has held consultations on the 1971 Sorcery Act and called for its repeal but has yet to present its report to the parliament, Pouilly said.
"We welcome the proposal by the Constitutional Law Reform Commission to repeal the Sorcery Act and we call for a stronger legal response to such killings," she told Reuters.