More than 1,000 are now feared to have died in a cyclone that smashed into Mozambique last week, while scores were killed and more than 200 are missing in neighbouring Zimbabwe.

The city of Beira in central Mozambique bore Cyclone Idai's full wrath on Thursday before the storm barrelled on to neighbouring Zimbabwe, unleashing fierce winds and flash floods and washing away roads and houses.

"For the moment we have registered 84 deaths officially, but when we flew over the area... this morning to understand what's going on, everything indicates that we could register more than 1,000 deaths," Mozambican President Filipe Nyusi said in a nationwide address.

"This is a real humanitarian disaster," he said. "More than 100,000 people are in danger".

Survivors have taken refuge in trees while awaiting help, the president added.

Aerial photographs released by a Christian non-profit organisation, the Mission Aviation Fellowship, showed groups of people stuck on rooftops with flood waters up to window level.

Cyclone Idai left 98 dead in Zimbabwe

"The scale of damage... (in) Beira is massive and horrifying", the International Federation of Red Cross and Red Crescent Societies (IFRC) said.

Some 90% of the city of 530,000 people and its surrounding area has been "damaged or destroyed," it said in a statement.

"The situation is terrible. The scale of devastation is enormous," an IFRC spokesman said.

"Almost everything is destroyed. Communication lines have been completely cut and roads have been destroyed. Some affected communities are not accessible."

A large dam burst on Sunday and cut off the last road to Beira, he said.

Sofala province’s governor warned that the "biggest threat we have now, even bigger than the cyclone, is floods because it's raining more and more".

The devastation is far greater than has been realised by the rest of the world, according to a spokesperson for the International Federation of the Red Cross.

Speaking on RTÉ's News at One, Matthew Coughran said entire communities have been washed away. 

"I think the sense of the team on the ground is this is a much bigger disaster than perhaps was anticipated, and certainly it is much bigger than the rest of the world has realised.

"We know the human toll is massive, we know there is at least 400,000 have been made homeless and that figure is probably higher. Tragically we heard yesterday that the death toll is in the region of 1,000 and that could climb in the coming days."

He said the city of Beira has been absolutely devastated by the storm.

"We estimate that 90% of buildings in the city were severely damaged or completely destroyed," he said. 

"We've also had reports from outside the city to the south and southwest where entire communities have been washed away. Where there used be villages there is now an inland sea that is as deep as six metres in some parts."

In neighbouring Zimbabwe, Cyclone Idai left 98 dead and at least 217 more missing, according to the information ministry.

The storm swept away homes and ripped bridges to pieces, leaving destruction that the defence minister said "resembles the aftermath of a full-scale war".

Some roads were swallowed up by massive sinkholes, while bridges were ripped to pieces by flash floods.

"This is the worst infrastructural damage we have ever had," Zimbabwe's transport and infrastructure minister said.