Veronique Taveau, a UNICEF spokeswoman, told journalists that 127 fresh cases had just been discovered in the village of Ishasha, north of Goma and near the Ugandan border.
She said aid agencies could not get to many areas because of the fighting health experts could not assess the scope of the outbreak. "There are numerous places we can't reach, which definitely makes us fear a sizeable rise in the number of cases," she said.
"It is vital that humanitarian access is allowed," she added.
The cases "seem to have spread very quickly due to a lack of hygiene and water," she said.
The World Health Organisation warned last week that cholera poses a "serious risk" in the region where around 1,000 cases have been reported since October.
The UN Office for the Coordination of Humanitarian Affairs also warned there was a growing risk of malnutrition, with 78 severe and acute cases reported in the town of Masisi over the weekend.
Fighting between the DR Congo government and followers of renegade Tutsi general Laurent Nkunda has killed at least 100 civilians and displaced more than 250,000, the UN and humanitarian groups have said.