A US-based pressure group says a recent massacre at a refugee camp in Burundi in which 160 people died was carried out by a Burundian rebel movement.
Refugees in Gatumba camp fled fighting in DR Congo in June
A preliminary report by the United Nations last week said that Congolese-based groups were involved in the attack.
But Human Rights Watch said this conclusion had helped threaten a renewed outbreak of war in the region.
It warned the UN to be "incredibly careful" in the volatile region.
The report says National Liberation Forces (FNL) rebels entered the Gatumba camp at night on 13 August and raked the refugee tents with gunfire.
They also threw incendiary grenades burning dozens of refugees to death.
HRW says the FNL rebels targeted tents occupied by the Banyamulenge - Congolese Tutsis - and left untouched nearby tents of Burundian Hutus.
The organisation says the Banyamulenge had been housed in white tents while the Burundian refugees occupied green tents.
The massacre led to threats of retaliation and even war against DR Congo from the governments of both Burundi and neighbouring Rwanda - despite the FNL claiming responsibility for the attack.
The report's author, Alison Des Forges, a respected expert on central African affairs, said the massacre was a war crime and should be dealt with by bringing the perpetrators to justice, not by more violence.
Rwanda and Burundi have invaded DR Congo in the past, in search of the Hutu rebels blamed for killing thousands of Tutsis in the last decade.