President Filipe Nyusi has said that the death toll in Mozambique from Cyclone Idai could be as high as 1,000.
Mr Nyusi flew over some of the worst-hit areas on Monday. He described sighting bodies floating in the rivers.
The storm made landfall near the port city of Beira on Thursday with winds of up to 177 km/h (106 mph), but aid teams only reached the city on Sunday.
A UN aid worker told the BBC that every building in Beira – home to half a million people – had been damaged.
Gerald Bourke, from the UN’s World Food Programme, said: “No building is untouched. There is no power. There is no telecommunications. The streets are littered with fallen electricity lines.
“The roofs on so many houses have fallen in, likewise the walls. A lot of people in the city have lost their homes.”
The official death toll in Mozambique stands at 84 following flooding and high winds. The cyclone has killed at least 180 people across southern Africa.
The International Federation of Red Cross and Red Crescent Society (IFRC) described the damage as “massive and horrifying”.
In Zimbabwe, at least 98 people have died and 217 people are missing in the east and south, the government said.
The death toll included two pupils from the St Charles Lwanga boarding school in the district of Chimanimani, who died after their dormitory was hit when rocks swept down a mountain.
Malawi was also badly hit. The flooding there, caused by the rains before the cyclone made landfall, led to at least 122 deaths,according to our source.
The UK government said it would provide humanitarian aid worth £6m ($8m)to Mozambique and Malawi. It also said it would send tents and thousands of shelter kits to Mozambique.