At least 36 people have died and nearly 70,000 have been displaced because of flooding in Mozambique, the UN says.
The number of people affected by the flooding could reach 100,000 as flood waters continue to rise in the coastal city of Xai-Xai, the UN added.
The UN said it would appeal to its donors for additional funds to deal with the emergency.
Days of torrential rains across the south-east of Africa have caused sea levels to rise to dangerous levels.
Neighbouring South Africa, Zimbabwe and Botswana have also been hit by severe flooding.
The United Nations in Mozambique said in a statement that 36 people had died so far across the country - 26 of them in the worst hit province, Gaza, in the south.
Some 65,000 people in Gaza alone had been affected by the floods, with nearly 50,000 seeking refuge in six temporary shelters in the worst-hit districts of Chokwe and Guija.
Overall, nearly 85,000 people have been affected by the floods and 67,995 have been temporarily displaced, the UN said.
"Together with government, we are rushing in clean water, food, shelter, and humanitarian supplies to Gaza Province, and are ready to send more as needs become clearer," Jennifer Topping, UN Humanitarian Coordinator for Mozambique, said.
The UN has staff on the ground in the worst-affected areas where food distribution has begun, and is setting up water supply structures, but Ms Topping said that: "We will be appealing to our donors to make additional funds available immediately to help deal with this emergency."
But it appears that aid has not yet reached many of the displaced. An AFP reporter in Gaza saw tens of thousands of people camping out at roadsides, some forced to eat grasshoppers to survive.
And officials are warning of a looming disaster in the city of Xai-Xai, with waters as high as eight metres (26 ft) expected to hit.
Severe flooding in Xai-Xai would sever the main road connection between the north and south of the country, the AFP reports.
Floodwaters in South Africa have claimed several lives and left hundreds stranded after the Limpopo river burst its banks on Monday.
A crocodile farm in the far north of South Africa was forced to open its gates because of the flooding, letting loose some 15,000 crocodiles - only a few thousand of whom have so far been found.