More than a thousand people are 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".

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.

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

Ninety per cent of the city of some 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," the IFRC's Jamie LeSueur 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 governor Alberto Mondlane warned that the "biggest threat we have now, even bigger than the cyclone, is floods because it's raining more and more".

'A PERFECT STORM'

Emma Beaty, coordinator of a grouping of NGOs known as Cosaco, said: "We've never had something of this magnitude before in Mozambique".

"Some dams have broken, and others have reached full capacity, they'll very soon open the flood gates. It's a convergence of flooding, cyclones, dams breaking and making a potential wave: everything's in place so we get a perfect storm."

Nyusi said the Pungwe and Buzi rivers in central Mozambique "have burst their banks and engulfed entire villages."

"Communities are isolated and bodies are floating" on the waters, he said.

Beira international airport was closed because of cyclone damage but later reopened.

ZIMBABWE HIT 

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

Families started burying their dead on Monday in damp graves, according to an AFP photographer.

The storm swept away homes and ripped bridges to pieces, leaving destruction that the acting defence minister, Perrance Shiri, 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 Infrastructural Development Minister Joel Biggie Matiza said.

The eastern district of Chimanimani was worst-hit, with houses and most of the region's bridges washed away by flash floods.

The most affected areas are not yet accessible, and high winds and dense clouds have hampered military rescue helicopter flights.

Two pupils and a worker at a secondary school in the area were among those killed after a landslide sent a boulder crashing into their dormitory.

Soldiers on Sunday helped rescue the surviving nearly 200 pupils, teachers and staff who had been trapped at the school in Chimanimani.

Joshua Sacco, lawmaker for Chimanimani, told AFP that between "150 to 200 people" are missing.

The majority of them are thought to be government workers, whose housing complex was completely engulfed by raging waters. Their fate was unknown because the area was still unreachable.

Zimbabwean President Emmerson Mnangagwa cut short a visit to Abu Dhabi returning home on Monday.

"With every hour and day that passes, our worst fears become increasingly real," he said in a statement "Many drowned while others were killed in their sleep from swift and unexpected rockfalls which demolished their homes".

His government has come under fire for failing to evacuate people in time.