The authorities in Malawi say at least 170 people have died in flash floods - a sharp rise on previous figures.
Heavy rain over the past month has swept many houses away and caused residents to flee to higher ground, some crossing the border to Mozambique.
Vice-President Saulos Chilima said more than 100,000 people had been displaced from their homes, mostly in the south.
Earlier this week, the government declared a third of the country to be a disaster zone and appealed for help.
The Malawian authorities have been using military helicopters and boats to reach some of those stranded.
But Mr Chilima said the rescue operation had been hampered by poor weather in the past few days and the difficulty of finding places for helicopters to land.
At the scene: Emmanuel Igunza, BBC Africa, Blantyre
Large areas of land in southern Malawi are now completely submerged. The on-going rains and damaged infrastructure - some key roads and bridges have been washed away - are further complicating rescue operations.
Rescuers are also trying to find bodies of people believed to have drowned in the Shire River. Some people have also been reluctant to leave their farms and now local elders have been tasked with ensuring everyone is taken to camps on higher, dry land.
But while the rescue efforts have concentrated on far-flung areas, much closer to Blantyre city, hundreds of families are now taking shelter in classrooms after their homes were destroyed by raging waters.
In Chilobwe township, massive boulders washed in from the hills are scattered everywhere - evidence of their destruction can be seen by the many semi-permanent brick houses smashed to the ground. Water is everywhere and many people have nowhere to go.
The vice-president said more helicopters and boats were needed, as well as tents and food.
South African troops are being deployed in neighbouring Mozambique where at least 25 people have died and a major road has been destroyed.