Christian militias killed as many as 30 civilians over the weekend in escalating violence on the Central African Republic border with Congo.
Many of those fighting are believed to be child soldiers who have been drugged.
Dozens of surviving civilians have fled into sanctuary inside a mosque in Bangassou, a town on the border, according to United Nations staff on the ground.
The Chrisitian militias are targeting Muslims in the violence, which has also seen attacks on the United Nations base and follows the departure of Ugandan and French peace-keeping troops.
At least six UN peacekeepers have been killed.
According to the local leader of the Red Cross, Pastor Antoine Mbao Bogo, the fighting has continued the point where the Red Cross and other aid organisations are unable to go in to help the wounded and retrieve the dead.
Herve Verhoosel, spokesman for the UN mission, told Reuters: 'The situation is extremely deplorable and we are doing everything to rapidly retake control of Bangassou.'
A disarmament programme put in place by the government has failed to halt the rising violence.
Prime minister Simplice Sarandji condemned the attacks.
The latest sectarian violence began four years ago after Muslim troops seized control of the former French colony.
Additional reporting by Reuters
