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 is 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. Medical agencies are now 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. BBC
