The number of people killed in Mozambique by powerful storms and floods across southeastern Africa is likely to rise significantly, the Red Cross said on Tuesday.

Rescue crews are still struggling to assess the devastation caused by Cyclone Idai which swept in at speeds of up to 170 kph (105 mph) from the Indian Ocean late last week, hitting Mozambique, then its inland neighbours Zimbabwe and Malawi.

The official death count in Mozambique stands at 84 - but its president Filipe Nyusi said on Monday he had flown over some of the worst-hit zones, seen bodies floating in rivers and now estimated more than 1,000 people may have died there.

A total of 98 people were confirmed dead and more than 200 missing in Zimbabwe, its government said on Monday.

The storm hit land near Mozambique's port of Beira on Thursday and went on to flood huge areas of territory, destroy roads, and wipe out communications networks across the region.

Drone footage posted online by the International Federation of Red Cross and Red Crescent Societies showed buildings in the coastal settlement of Praia Nova, outside Beira, flattened by storm winds.

"We are working with NASA and the European Space Agency to get satellite information to get a full picture of the affected areas and number of people trapped there," Caroline Haga of the International Federation told Reuters.

"Given the sizes of these areas we expect the death toll to increase significantly."

People were still trapped on high-lying areas of the country, Gerald Bourke of the United Nations World Food Programme said.

"We don't have any clear numbers on the death toll but we are looking at huge areas that are under water. We're seeing mile after mile of villages under several metres of water," Bourke said.