MAPUTO, Mozambique (AP) -- Floodwaters rose rapidly today in central Mozambique, threatening to sweep away thousands of people clinging to trees and perched on rooftops with their hopes of rescue fading fast.

Three weeks after torrential rains began drenching southern Africa -- and four days after Cyclone Eline roared through -- the biggest human tragedies are still unfolding. Aid workers, hampered by washed-out bridges and roads everywhere, are not able to cope with the scale of the disaster.

Aid workers who flew over the Save River Valley saw thousands of people in trees, on roofs and on narrow strips of land, apparently marooned since Monday. The government estimated 17,000 people in the valley were in danger.

``We need helicopters today. These people are very, very desperate. It must be a nightmare for them,'' said Carol Collins of the British charity Save the Children. ``The water is moving very, very rapidly. I'm sure some of them were washed away.''

Rain fell again today as another tropical storm developing over the Indian Ocean started moving toward Mozambique.

The Mozambican government estimated more than 200,000 people were homeless and at least 70 have died so far. Similar death and homeless figures were reported in neighboring Zimbabwe and South Africa.

In Zimbabwe, floodwaters Friday night shoved a bus carrying an unknown number of passengers off a bridge and into a raging river. No survivors were found today.

In addition, A family of seven drowned when their hut was swept away in southern Zimbabwe and six tourists were missing after trying to reach the banks of a river when their bus got stuck on a flooded bridge. Their identities or nationalities were not immediately available.

Some areas of South Africa's Northern Province were completely cut off, said Col. Flip le Roux.

``There must be far more people in danger than we know of,'' Le Roux told five Cabinet ministers who flew over the area today.

Bridges were broken like twigs by the force of floodwaters. Roads were converted into streams. Streams had become rivers.

Le Roux said a mass of water was on its way to the already swollen Limpopo River, which divides South Africa and Zimbabwe and then spills into southern Mozambique.

South African Air Force helicopters have plucked more than 3,000 people to safety in southern Mozambique over the past two weeks. But Maj. Louis Kirsten, an air force spokesman, said today there were no plans to fly to the Save River Valley, which was out of their range.

Several trucks carrying emergency aid became stuck Friday in the rising flood waters near the town of Save (pronounced sah-VAY).

The World Health Organization has warned that 800,000 were at risk from cholera, malaria and other diseases as a result of the floods.

The government and U.N. agencies said they have received a strong response to an international appeal for $65 million in aid. A U.S. Air Force plane carrying tents, blankets and other humanitarian supplies was due in the capital, Maputo, on Monday.