Hutu rebels and allied attackers armed with guns and machetes killed at least 159 people in "a plan of genocide" at a camp for Tutsi Congolese refugees in western Burundi, the army said on Saturday.
Women screamed and wailed at the horrific scenes of bodies of men, women and children hacked down or shot dead. A witness saw one woman lying dead with machete wounds all over her body as her infant child clung at her breast.
The Hutu Forces for National Liberation (FNL) claimed responsibility for Friday night's attack, saying they were aiming to hit a military camp nearby.
But Burundi's army said the attack's purpose was killing refugees at their camp and it was not aimed at the military base. Six bodies of refugees who had been kidnapped and later killed were recovered near the camp, it said.
Isabelle Abric, spokeswoman for the United Nations mission to Burundi, said 153 people had been killed in the camp, and another 30 among those taken to hospital after the attack may have died.
The UN refugee agency said it would consider moving the camp where the army said 111 people had also been wounded.
Some refugees said the attack was planned, citing unsigned leaflets circulated this week urging death to the Banyamulenge - the term for the mostly Tutsi Congolese refugees.
Burundi's President Domitien Ndayizeye visited the UN refugee agency's Gatumba camp, which is near the town of Uvira in eastern Democratic Republic of Congo and provides shelter for between 2,500 and 3,000 Banyamulenge.
Democratic Republic of Congo (DRC) President Joseph Kabila called for an international inquiry into the killings.
Thousands of Banyamulenge refugees have taken shelter in camps in Burundi run by the United Nations refugee agency after fleeing the DRC in the past few months saying they were terrified of being targeted by government troops, local militia and Congolese civilians in eastern Congo.
The 3,000-3,500-strong FNL is the only rebel group refusing to join a power-sharing government to end a decade-long civil war between Burundi's politically dominant Tutsi minority and rebels from the Hutu majority, in which 300,000 people have been killed, mostly through hunger and disease.