The early heavy rains have swollen rivers to alarming levels across the region, catching authorities off guard and forcing governments and aid agencies to step up efforts to avert crisis.

"At least 1.5 million will be displaced by the floods and the government and aid groups will have to provide relief food and shelter to the families in tents for some time," said a senior Zambian government official who wished not to be named.

Waters that had reached a depth of six metres (18 feet) forced some people to seek refuge on trees and rooftops in Mozambique, where the United Nations said it would take urgent measures to help victims of the floods.

The UN noted an estimated 56,000 people had been affected, including 13,000 who had been relocated to resettlement centres, after heavy rains led to a sharp rise in water levels on the Zambezi, Pungue, Buzi and Save rivers.

"Governments and international humanitarian organizations are scaling up their efforts to ensure a swift response and save lives," said John Holmes, United Nations Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator said in a statement issued in New York and Johannesburg.

In early 2007, floods in central Mozambique killed 45 people and left 285,000 homeless, while cyclone Favio displaced another 140,000 people.

It was the worst flooding to hit the former Portuguese colony since floods in 2000-2001 killed 700 people and drove half a million from their homes.

International aid agencies also expressed worry over erratic weather patterns in southern Africa, which have devastated harvest prospects for millions of people.

"We are greatly concerned at the emergency responses this early in the rainy season," said Kelly David, Head of the United Nations Office for the Coordination of Humanitarian Affairs for Southern Africa.

"If this continues, we can expect a substantial impact."

Guy Robinson, president of umbrella farmers group the Zambia National Farmers Union (ZNFU), told Reuters heavy rains had wiped out some plantings. He said the most affected area was southern Zambia, one of the country's major farming regions.