Some 15,000 people still need to be rescued from the devastation caused by Cyclone Idai, Mozambique officials say.
The cyclone victims there are stranded by catastrophic flooding and are clinging to roofs or stuck in trees, charities say.
In the port city of Beira, aid workers say there are only two to three days of clean water left.
Some 300 people are confirmed dead in Mozambique and Zimbabwe, but the toll is expected to rise.
The powerful cyclone swept in to Beira last Thursday, with winds of more than 177km/h (106 mph). It left a trail of devastation as it moved inland.
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Oxfam has told the BBC that an area of about 3,000 sq km (1,864 sq miles) is now under water.
Medical agencies are warning that the shortage of food and clean drinking water is increasing the threat of disease.
“The first thing you see when you arrive is destruction and a lot of water,” said Get Verdonck, an emergency co-ordinator with the medical charity Médecins Sans Frontières (MSF).
“People are using well water with no chlorination, and that water is unlikely to be clean… pneumonia and other respiratory diseases are going to be a problem,” he told Reuters from Beira.
Aid groups said Mozambique had borne the brunt of flooding from rivers that flow downstream from neighbouring countries.
A total of 217 people are confirmed dead in the country, but many areas have still not been reached.
Caroline Haga, an official with the International Federation of Red Cross and Red Crescent Societies, said the worst-hit areas were close to the Buzi river west of Beira.
She said rescue teams were dropping high-energy biscuits, water purification tablets and other supplies to people surrounded by water and mud.
“We have thousands of people… in roofs and trees waiting for rescue,” Ms Haga told AFP news agency.
“We are running out of time. People have been waiting for rescue for more than three days now. We can’t pick up all the people so our priority is children, pregnant women, injured people.”
Celso Correia, Mozambique’s minister of land and environment, confirmed this, telling Reuters the number of people still needing to be rescued was thought to be about 15,000.
He added that 3,000 people had already been rescued so far.