The sites were found in the Gasabo district, outside the capital Kigali, and about 200 bodies have been exhumed.

Around 3,000 people from the area went missing during the massacres, and local people believe the graves may contain all of their bodies.

Some 800,000 people - ethnic Tutsis and moderate Hutus - were slaughtered in 100 days by Hutu militias.

The graves were uncovered two weeks after commemorations were held to mark the start of the killings.

Volunteers are leading the search after being told of the location of the graves by a woman, who claims to have seen bodies dumped there.

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"The exercise is ongoing as we have identified four mass graves," Théogene Kabagambire, an official with the genocide charity Ibuka, told News Day, a Rwandan newspaper.

Many of the genocide's perpetrators have been released from prison after having completed their sentences.

BBC Africa security correspondent Tomi Oladipo says the discovery has raised questions in the local media about why the people who knew about these sites have held back from revealing their locations.

Mr Kabagambire says they are still searching for a fifth grave, and adds that some genocide convicts "are doing little to reveal the whereabouts of our loved ones".

Relatives of genocide victims have been scouring through the sites in search of their loved ones' remains.

"I have information that both my parents were killed and dumped in one of [the] mass graves here and I came with hope that I can identify the clothes they were wearing when they left," Isabelle Uwimana, one of survivors, told News Day.

"I wish to be sure that they are here so that I give them a decent burial."