Tribal fighters who boarded wooden canoes in the Democratic Republic of Congo have massacred 100 men.
A UN spokeswoman said women had also been raped in the attack on Lake Albert.
"We got this information from interviewing two survivors including one who said he hid under dead bodies to save his life" said Ms Isabelle Abric, spokeswoman for the US mission in Congo.
However, the accuracy of the death toll is in doubt "because these people were busy saving their lives", she said.
The slaughter took place on January 15th, a day after two dozen Lendu tribal fighters boarded five of the canoes - the main form of transport - saying they would protect the 180 passengers.
However, the fighters ordered the canoes to stop and killed most of the male passengers and ordered the women and children to carry cargo looted from the vessels, Ms Abric said.
The UN has sent "troops and human rights officials to the area to investigate exactly what happened" and determine casualty figures, said Major General Samaila Iliya, the Nigerian commander of the UN force in Congo.
The area remains volatile as Lendu and Hema tribal fighters battle each other as well as the UN troops trying to stem the bloodshed months after peace deals ended major fighting. The five year civil war in Africa's third-largest nation has claimed an estimated three million lives.