The president of Mozambique says the high water has displaced a million people, and warns international aid sent to relieve devastating flood damage has been slow to arrive.
Mozambique President Joaquim Chissano says that after flying over affected areas, it was clear that more help was needed, and fast.
"We say thank you for all the help we have received but we are asking for more. Our people have nothing and the world can do more," Chissano added.
More high water is expected by week's end.
Some of them are going into their fourth day in the open, clinging desperately to life and in dire need of food and clean water.
A giant U.S. C-17 transport plane arrived Wednesday morning with enough plastic sheeting for 10,000 people, 6,000 water jugs and, most important for those trapped, high-protein biscuits.
``These materials will be distributed to the victims across Mozambique with the assistance of the U.N. World Food Program,'' U.S. embassy spokesman Steven Koenig told reporters.
The U.S. embassy said Washington was sending an additional seven-man team to coordinate further U.S. help to this impoverished southern African nation.
World Food Program spokeswoman Brenda Berton said Britain had agreed to fly four large helicopters to Maputo to help the small fleet of five South African helicopters at work for the past three weeks.
She said the United States was sending a further two helicopters, effectively doubling the fleet that has rescued more than 8,000 people in four days.
Washington said on Tuesday it would donate an extra $7 million worth of food to Mozambique and $3 million in other aid.
Mozambique's water authority has warned that fresh flooding from Botswana and Zimbabwe will hit the ravaged central and southern regions of Mozambique in the next few days.
The South African Air Force has rescued 8,000 people since the latest wave of floods swelled on Sunday, but aid workers estimate that about 100,000 are still stranded.
Aid workers say they expect that number to rise sharply as disease spreads among thousands of displaced people.
``We are not giving any estimates of fatalities now,'' said Ian MacLeod, emergency coordinator for the U.N. Children's Fund.
``But based on what we are seeing on the ground, at the end of the crisis, the deaths are likely to run into the thousands because of the problem of sickness and disease due to a large number of populations concentrated together,'' he said.
Greg Hartl, spokesman of the World Health Organization (WHO) in Geneva, said that flooding and contamination of drinking water supplies increased the risk of outbreaks of disease.
"Cholera is a big concern. Malaria wil become an increasing concern as flood waters recede and stagnate. Dysentery, obviously, is already a concern," Hartl said.
Neighboring Zimbabwe said it needed at least $7.9 million to repair flood-damaged infrastructure and warned that thousands of people there faced starvation because food distribution had been virtually impossible in some areas.
The U.N. Office for Coordination of Humanitarian Affairs said several donor countries had pledged $13.5 million to Mozambique at emergency talks in Geneva. The government has said it will need $65 million to rebuild flood-stricken areas.
Neighbouring Botswana said it would give Mozambique one million litres of fuel to help in rescue operations.
The World Food Program said it had launched a $6.8 million emergency aid operation and estimated that immediate help was needed for up to 300,000 people.
The WFP has said it will use its own cargo planes and hire private aircraft to join South African aircraft and helicopters which have already distributed 1,200 tonnes of food for the WFP.
Britain said it had sent a military reconnaissance team to Mozambique to assess how it could help, and announced it would cancel all Mozambique's debts to it.