Some 29 people have been killed by suspected Rwandan Hutu rebels in eastern Democratic Republic of Congo, aid workers say.
The FDLR want an amnesty before returning to Rwanda
Victims were slashed with machetes during the night attack.
Seventeen bodies were found in three villages in the South Kivu province and another 12 found in the forest later, said UN spokesman Samuel Zungrana.
Democratic Liberation Forces of Rwanda (FDLR) leader Ignace Murwanashyaka told the BBC his men were not involved.
Some of those who part in Rwanda's 1994 genocide fled across the border into eastern DR Congo after the Hutu government was ousted from power, leading to instability across the region.
The Congolese army and UN peacekeepers have been carrying out joint operations in the area against the fighters since January.
The UN Mission in DR Congo (Monuc) says it is investigating the reports of the attack in Kanyola district.
"They've just begun burying the dead. Everyone is traumatised. Everyone is afraid," local aid worker Constantin Charondagwa told Reuters news agency.
Mr Murwanashyaka said it was impossible to identify who was behind the attack, as it happened at night.
The rebels have repeatedly said they do not want to go back to Rwanda unless they are granted amnesty.
The Tutsi-led government in Rwanda has twice invaded Dr Congo, saying it wants to wipe out the FDLR.
They include members of Rwanda's former army and extremist Hutu militia, the Interahamwe, who led the 100-day genocide where 800,000 Tutsis and moderate Hutus were killed.
Their presence has also led to tensions between DR Congo's own Hutu and Tutsi population.
Former Congolese Tutsi rebel General Laurent Nkunda refuses to fully integrate his forces into the army, citing a need to protect the Tutsi population.
Last year's historic elections were supposed to mark the end of years of conflict and mismanagement in DR Congo.