Gunmen have killed at least 156 people - mainly women and children - in an overnight raid on a camp for Congolese Tutsi refugees in Burundi, the UN says.
The refugees were escaping violence in the DR Congo
The Gatumba camp, near the border with DR Congo, provides shelter for 1,700 refugees who fled the country in June.
Men armed with machetes and guns attacked the camp, torching houses and leaving the scene littered with bodies.
A Hutu rebel group has claimed responsibility for the attack, which took place after nightfall on Friday.
Witnesses described finding charred and bloody bodies at the camp for the Tutsis escaping recent violence in the Democratic Republic of Congo.
"People were sleeping when the attack happened. People were killed as they tried to escape," Eliana Nabaa of the UN mission in Congo said, quoted by the Associated Press news agency.
"The scene is absolutely horrific. There are many people burnt - families, children, women and men burnt," she said. The camp is one of several run by the UN refugee agency near the border between the two countries.
UNHCR spokesman Fernando del Mundo told BBC News Online that witnesses reported hearing the beating of drums before the attack.
He said 136 people died on the spot and 20 later in hospital, while scores of others were injured.
He said the UN was now moving the surviving refugees to a nearby school.
Burundi's President Domitien Ndayizeye, expressed his shock after surveying the scene. "This is unbelievable, I have never seen this before," he told Reuters news agency.
He said the massacre had been carried out by Congolese who had crossed into his country.
The Hutu Forces for National Liberation (FNL) group claimed responsibility for the violent attack, saying it had aimed to hit a military base, some 500m from the Gatumba camp.
"We then heard gunshots coming from the refugee camp aiming at us and then decided to retaliate," said FNL spokesperson Pasteur Habimana, quoted by Reuters.
The UNHCR confirmed that the military base had been attacked.
Burundi's army denied that shots had been fired from inside the refugee camp, saying the rebels' attack was deliberate.
Some of the refugees, the Banyamulenge who fled the Sud-Kivu region in eastern DR Congo, also said the attack on the camp was planned.
They cited unsigned leaflets circulated in the camp and urging death to the Banyamulenge.
The attack followed a two-day visit by one of the DR Congo's four vice-presidents, himself a member of the Tutsi community, to Burundi to speed up the process of repatriating the estimated 20,000 Congolese refugees.
Burundi - like neighbouring Rwanda - has seen years of conflict between the Hutu majority and Tutsi minority, with large numbers of each community crossing into neighbouring countries to escape ethnic violence.