The Rwandan Government has been urged to halt the murder and intimidation of potential witnesses to the genocide in 1994 during which 800,000 people died.
Memories of the genocide remain fresh, almost 10 years on
An organisation representing survivors of the genocide, Ibuka, says a number of people have been killed this year.
Spokesman Benoit Kaboyi says witnesses are being silenced in an attempt to undermine a rural justice system introduced in Rwanda 18 months ago.
Gacaca courts are meant to clear the backlog of pending genocide trials.
Ibuka said one or two genocide survivors are killed every month but it says three potential witnesses were killed recently in the south west province Gikongoro.
Ibuka says in the most recent case a man was killed and dismembered in front of his family as a warning to other potential witnesses.
"The ultimate reasons behind the killings is to block and scare away genocide survivors from testifying in gacaca courts," Ibuka said.
"These killings are well planned and are targeting one section of people with an intention of keeping their lips shut."
Police spokesman Damas Gatare has told the BBC that two murders had occurred in Gikongoro, but he said these were isolated cases and denied that there was an increase in threats to witnesses since the start of Gacaca trials.
He said a number of suspects had been detained in connection with the murders.
Some 100,000 suspects have still to go on trial, accused of participating in the killings of Tutsis and moderate Hutus over 100 days in 1994.
Officials in Rwanda say that the gacaca system will be expanded across Rwanda next year.