"They speak of men, in particular, being targeted and beheaded and many, many reports of women and children ... being kidnapped or simply disappearing," a UNHCR spokesperson told a briefing in Geneva
With less than 15% of the country's citizens connected to electrical infrastructure, most people "face chronic poverty" in the region with little means to get themselves out of penury.
Cyclone Kenneth - packing storm surges and winds of up to 280 km per hour (174 mph) - struck on Thursday night, destroying almost all the homes on the island.
Aid agencies said they were still struggling to reach victims and work out the extent of the devastation in the northern province of Cabo Delgado.
The floods have left thousands of people scrambling for shelter, food and water.