The death toll in Mozambique from Cyclone Idai could be as high as 1,000, President Filipe Nyusi has said.
Mr Nyusi flew over some of the worst-hit areas on Monday. He described seeing 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.”
Rescue crews spent much of the night helping people from trees, Jamie LeSeur, the head of the IFRC assessment team, told the BBC.
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.
Local people in Beira have put in an “incredible effort” to reopen roads in the city, Mr LeSeur told the BBC’s Newsday programme.
The road linking Beira to the rest of the country was damaged, but air links have now resumed. President Filipe Nyusi cut short a trip to eSwatini, formerly known as Swaziland, to visit the affected areas.
What’s the situation in Zimbabwe?
A state of disaster has been declared in Zimbabwe. President Emmerson Mnangagwa has returned home early from a trip to the United Arab Emirates to “make sure he is involved directly with the national response”, the authorities say.
The ministry of information has shared pictures of pupils from St Charles Lwanga School, who have now been rescued.
Shocked survivors at a hospital in Chimanimani district told how the floods destroyed their homes and swept away their loved ones.
“I still have not found where my daughter is buried in the debris,” Jane Chitsuro told the AFP news agency. “There is no furniture, no more clothes, there is only rubble and stones.”
Praise Chipore’s house was also destroyed. “My daughter who was with me in bed was washed away from me and a bigger flood carried me further away,” she said.