At least 48 people have been killed and nearly 70,000 left homeless by severe flooding in Malawi, the country's president announced during an appeal for international assistance.

"Since the flooding started 13,999 households have been affected, 69,995 (people) rendered homeless, (and) 48 lives have so far been lost since," President Peter Mutharika said in a statement detailing "the national disaster" Tuesday.

The floods caused by heavy rains have stricken half of the southern African country's 28 districts -- most in the south of the nation, which is among the world's poorest.

In the shantytowns surrounding the commercial capital Blantyre, waters flattened houses and left a trail of mangled corrugated iron sheets, boulders, trees and litter.

"The floods came with fury and without warning," said Robin Phiri, a resident of the Chilobwe shantytown, which sits at the edge of a mountain overlooking the city.

His neighbour's house was demolished when a nearby stream overflowed.

The neighbour and his wife were pulled from the debris by community members, but their son -- a medical student -- is still missing.

"You can see the destruction," said Phiri. "There was a house where we are standing."

Roads and rail lines have been cut by flooded rivers, which according to officials have also swept away surprised bystanders, people fleeing in canoes and sleeping families in their homes.

"The floods are a national tragedy that urgently needs both local and international response," Mutharika said.

Meteorologists have predicted more rain.