Half a million people have already been affected by the floods
Vast areas of southern Africa have been hit by the worst flooding in 50 years, says the Red Cross.
With more than 100 people dead, an area 1,500km (930 miles) wide, from Namibia in the west to Mozambique faces rising waters from several rivers.
The Red Cross says it has only managed to reach a small fraction of the more than half a million people who have already been affected.
It is appealing for helicopters and boats to reach isolated communities.
Matthew Cochrane, from the International Federation of Red Cross, told the BBC that if Zimbabwe's Kariba dam reaches capacity, authorities there may be forced to open the floodgates.
If this happens, Mozambique would be hit by a wall of water and tens of thousands more people would be displaced, he warned.
Nearly 100,000 people have already been made homeless, he added.
He said the organisation expected the flood waters to remain for at least the next four to eight weeks.
Mr Cochrane said the waters had reached densely-populated communities - up to 30km (19 miles) from the Zambezi river - that had never been affected by flooding before.
"They've been hit with floods and they've got no natural reflex to evacuate, to protect their possessions, to find higher ground," he said.
"So the floods have been particularly difficult for them and there's a huge need for them to receive support."
The Red Cross said it had only reached about 35,000 people across the region with basic humanitarian aid, like mosquito nets, tarpaulins and water purification sachets.
It said heavy rain was continuing to fall in the Democratic Republic of Congo and northern Zambia.
This would wash more water down the Zambezi basin and could flood east to Malawi and Mozambique, it added.