UN spokeswoman Lindsey Davies said on Tuesday that at least 55 000 flood victims in Mozambique's southern district of Chokwe face dehydration or illness because of a shortage of drinking water.
She said that in Chokwe town in the Limpopo river valley, "conditions are appalling, with homes still in the mud and a horrible smell of dead people and animals."
Davies said local health authorities had been boiling water on fires made with wood, which was difficult to find, but this was not enough for thousands of people in the area.
"We have seen a severe shortage of drinking water at the Chihaquelane camp, the largest accomodation centre with about 45 000 people, and in Chokwe town where we are starting to see people returning to their homes".
Davies said about 11 000 people had already returned to the town, and an airlift of drinking water was needed, along with efforts to help people filter and purify water and get the pumping facilities back in operation.
Mozambican Interior Minister Almerinho Manhenje warned residents of Chokwe town, which had a population of about 60 000 before the floods, not to return to their homes "because the town is not yet habitable".
Nearly 1 000 US soldiers are expected to participate in the aid operation, with most based in the central port city of Beira, along with a dozen large helicopters.
More heavy rain is expected to fall over Mozambique over the next 24 hours.
Besides aid being provided by numerous international relief organizations, at least 16 Western and African governments have come to the rescue. Around $103-million has been placed in the relief coffers.