Flooding has began in parts of northern Mozambique that were hit by Cyclone Kenneth three days ago.
The government has urged many people to immediately seek higher ground, with hundreds of thousands of people at risk.
Authorities have said at least five people died after the storm arrived on Thursday evening with the force of a Category 4 hurricane, stunning residents of a region where a cyclone had not been recorded in the modern era.
Kenneth came just six weeks after Cyclone Idai ripped into central Mozambique and killed more than 600 people.
The remnants of Kenneth could dump twice as much rain as Idai did, the UN World Programme has said.
It was the flooding after Idai that caused most of the deaths.
Heavy rain was falling in the main city of Pemba, which had lost power.
As much as nine inches of torrential rain, or about a quarter of the average annual rainfall for the region, is forecast over the next few days.
Nearly 700,000 people could be at risk in the largely rural region, many left exposed and hungry as waters rise.
Some rivers in the region have burst their banks in the past, notably in 2000.
With streets nearly deserted in Pemba, a few braved the pouring rain. One woman held a plastic dish over her head, negotiating a flooded street.
“I have never seen such rains in my life, this doesn’t happen in Pemba. The storms we sometimes have, but such rains, never,” said a 35-year old resident, Michael Fernando.
Aerial photos taken on Saturday showed several coastal communities flattened by the storm in Mozambique’s northernmost Cabo Delgado province.
“Not a single house is standing anymore,” Saviano Abreu, a spokesman with the UN humanitarian agency, told reporters after the aerial assessment.
With many houses built of mud, wooden poles and metal roofs destroyed, families have begun wading through rising waters to what they hope are safer areas or huddling under impromptu shelters.
This was the first time in recorded history that the southern African nation has been hit by two cyclones in one season, again raising concerns about climate change.
The International Federation of Red Cross and Red Crescent Societies has reported heavy damage to Cabo Delgado province, with the communities of Macomia, Quissanga and Mocimboa da Praia of highest concern.
About 3,500 homes in parts of Cabo Delgado were partially or fully destroyed by the cyclone, with electricity cut, some roads blocked and at least one key bridge collapsed.