More than 150 refugees, mostly women and children, are being buried in mass graves in Burundi.
The refugees were escaping violence in the DR Congo
They are mostly ethnic Tutsis who fled neighbouring Democratic Republic of Congo in June. The Burundi authorities accuse armed groups from Burundi, Rwanda and DR Congo of being behind the killings.
BBC News Online looks at the background to the killings and the links between the conflicts in the three countries.
Who killed the refugees?
A Burundi rebel group - the National Liberation Forces (FNL) - has admitted responsibility. It says the Gatumba refugee camp was being used by armed Tutsi groups to attack its mostly Hutu forces.
The FNL says it carried out the attacks alone but the Burundi authorities have also accused the Rwandan Interahamwe and the Congolese Mai-Mai militias of involvement.
The Interahamwe are the remnants of those who carried out the 1994 genocide in Rwanda, in which some 800,000 ethnic Tutsis and moderate Hutus were killed. After the genocide, they fled to DR Congo, where they have been hiding ever since.
Leaflets were found in the camp, warning refugees to leave or face attacks by a coalition of Burundian, Rwandan and Congolese factions seeking "to fight the Tutsi colonisation in the region".
In the decade since the Rwandan genocide, Hutu rebels from Burundi and Rwanda have sometimes worked together, from their bases in DR Congo, along with armed Congolese factions.
Who are the refugees?
They are mostly Congolese Tutsis, called the Banyamulenge, who fled DR Congo after fighting in June between Tutsi rebels and the regular army around the town of Bukavu.
Tens of thousands fled after Tutsi General Laurent Nkunda withdrew from Bukavu, fearing reprisals.
The victims are being buried in mass graves
What is the link between the three countries?
Tutsi minorities have dominated both Burundi and Rwanda since pre-colonial times.
The Hutu majorities in both countries have been fighting for a greater share of power and wealth and bitter conflicts have followed.
There have been periodic massacres for generations and the Banyamulenge are descended from Tutsis who fled previous conflicts in Rwanda.
Rwanda and Burundi have both sent troops into DR Congo to hunt down the rebel groups based there and have accused previous Congolese governments of supporting them.
This led to a five-year conflict in DR Congo, which drew in at least six African countries.
Why does it matter?
At least three million people died in the DR Congo conflict, dubbed "Africa's first world war".
A deal was signed to end this war in 2002 but this year's fighting around Bukavu showed that tensions remain high.
Following the massacre, Rwanda has warned that it could again send troops into DR Congo, to disarm the Hutu militias.
This raises the prospect of a return to hostilities across much of central Africa.