Doraci Edinger had reportedly been strangled and beaten in her home in the northern city of Nampula.

The traffickers are said to target the sex organs of children, which are sold to make magic charms.

The nuns from the Sisters Servants of Mary Immaculate said they had received death threats since their report.

On Tuesday, the authorities said they had found no evidence of a trade in human organs.

But the nuns say they have spoken to victims who managed to escape the ring and have photos of dead children with missing organs.

'Powerful charms'

The BBC's Jose Tembe in the capital, Maputo, says that many people believe that a ring does exist and accuse the government of not doing enough to investigate it.

One nun told our correspondent that she was extremely angry at the news of the death in Nampula.

Many Africans believe in traditional medicine

These are believed by some to bring financial or sexual success to those who use them.

"Several countries are involved in this iniquitous game and the victims are the poor, those who have no voice or defence, or the strength to defend themselves, we are convinced that Nampula is part of an international ring," order spokeswoman Sister Juliana told Portuguese radio earlier this month.

She said there have been several attempts to abduct children from the orphanage they run in Nampula.

Mozambican, South African, Brazilian and Portuguese nationals were involved in the ring, she said.

The organs were reportedly being smuggled into neighbouring Zimbabwe and South Africa.