Aid agencies are trying to reach the survivors of Cyclone Idai in Mozambique
Charities say that thousands of people are stranded by catastrophic flooding, clinging to rooftops or stuck in trees.
In the port city of Beira, helpers say that they only have clean water for two to three days.
Some 300 people have been declared dead in Mozambique and Zimbabwe, but the number is expected to increase.
Food shortages and clean drinking water increase the risk of disease, medical authorities warn.
The mighty hurricane came in Beira last Thursday with winds of more than 1
"The first thing you see when you arrive is destruction and a lot of water," said Get Verdonck, an emergency coordinator for the medical aid organization Médecins Sans Frontières (MSF).
"People use well water without chlorination and it is unlikely that water is clean … pneumonia and other respiratory diseases will be a problem," he told Reuters of Beira.
Aid groups said Mozambique had borne the brunt of floods from rivers that flowed from neighboring countries. At least 200 people are known to be dead in the country, but many areas have not yet been reached.
Caroline Haga, an official of the International Federation of Red Cross and Red Crescent Societies, said the most affected areas are near the Buzi River west of Beira.
She said rescue teams dropped energy-saving biscuits, water purification tablets, and other supplies of people surrounded by water and mud.
"We have thousands of people … in roofs and trees waiting to be rescued," said Ms. Haga told AFP.
More about Cyclone Idai:
"We do not have time anymore, people have been waiting for rescue for more than three days, we can not bring everyone to the fore – children, pregnant women, injured people."  Image copyright
Deborah Nguyen from the World Food Program (WFP) said the priorities would reach those trapped in the flooded areas and then organize temporary shelter for the rescued.
"The relief is making progress, but there is still a lot," she told AFP.
Mozambique's President Felipe Nyusi said more than 100,000 people are at risk.
What happened elsewhere?]
At least 98 people died in Zimbabwe and 217 people were missing in the East and South, the government said.
In Malawi, the UN says more than 80,000 people have been displaced by the cyclone.