Hundreds of thousands of people scrambled for shelter, food and water across a swathe of southern Africa on Friday after a cyclone claimed more than 500 lives and swept away homes and roads, testing relief efforts for survivors.

Cases of cholera have been reported in Mozambique's cyclone-hit port of Beira, the International Federation of the Red Cross and Red Crescent Societies (IFRC) said in a statement.

"There is growing concern among aid groups on the ground of potential disease outbreaks. Already, some cholera cases have been reported in Beira along with an increasing number of malaria infections among people trapped by the flooding," the statement said.

Cholera is spread by feces in sewage contaminating water or food, and outbreaks can develop quickly in a humanitarian crisis where sanitation systems are disrupted. It can kill within hours if left untreated.

Cyclone Idai killed 242 people in Mozambique and 259 in Zimbabwe, and numbers were expected to rise, relief agencies said. In Malawi, 56 died in heavy rains before the onset of Idai.

As survivors gathered in informal camps and health officials warned of growing danger from measles and cholera, UNICEF executive director Henrietta Fore said the situation on the ground was critical, with no electricity or running water.

"Hundreds of thousands of children need immediate help," she said, estimating 1.7 million people were affected by the storm.

Even as floodwaters began to recede in parts of Mozambique, fears rose that the death toll could soar as bodies are revealed.

The number of deaths could be beyond the 1,000 predicted by the country's president earlier this week, said Elhadj As Sy, the secretary-general of the International Federation of Red Cross and Red Crescent Societies.

The city of Beira has become the centre for frantic relief efforts in Mozambique. (Cara Anna/Associated Press)

In addition to worries about the number of dead, As Sy told The Associated Press that the humanitarian needs are great.

"I fear we will be seeing more in the weeks and months ahead, and we should brace ourselves," As Sy said.

Thousands of people were making a grim voyage toward the city of Beira, which suffered massive destruction but has still become a centre for frantic rescue efforts throughout the region.

Some walked along roads carved away by the raging waters a week ago. Others, hundreds of them, were ferried in an extraordinary makeshift effort by local fishermen who plucked stranded people from small islands.

Helicopters set off into the rain for another day of efforts to find people clinging to rooftops and trees.

'Horrendous' situation

For those who reach Beira with their few remaining possessions, life is grim. Waterborne diseases are a growing concern as water and sanitation systems were largely destroyed.

"The situation is simply horrendous, there is no other way to describe it," As Sy said after touring transit camps for the growing number of displaced. "Three thousand people who are living in a school that has 15 classrooms and six, only six, toilets. You can imagine how much we are sitting on a water and sanitation ticking bomb."

What moved him the most was the number of children without their parents, separated in the chaos or newly orphaned.

"Yesterday [we] did a reconnaissance and we found another [inland] lake. So we are still very early in the phase of identifying what the scope of this is, for who is affected and how many are lost," Emma Batey, co-ordinator for the consortium of Oxfam, CARE and Save the Children, told the AP.

Luckily, the area is a national park and less densely populated, she said. Still, "there were devastatingly small amounts of people."

She estimated that another 100 people would be airlifted out on Friday: "We're only picking up those in absolute dire need."

No one is still clinging to roofs and trees, she said.

This map shows the extent of the flooding on Thursday. Rescue efforts continued Friday, as teams try to determine how many people are in need of help.

Pedro Matos, emergency co-ordinator for the World Food Program, said that what rescuers are seeing now is "sometimes it's just a hut completely surrounded by water."

"If islands are big enough, we can even see smoke coming out, meaning that they're cooking," he said, adding that it remains "super difficult" to estimate a death toll or even the number of missing.

For residents of Beira, life staggered on. People salvaged the metal strips of roofs that had been peeled away like the skin of a fruit. Downed trees littered the streets. And yet there were flashes of life as it used to be. White wedding dresses stood pristine behind a shop window that hadn't shattered.

Extent of Zimbabwean damage emerging

Zimbabwe was also affected by the cyclone and as roads began to clear and some basic communications were set up, a fuller picture of the extent of the damage there is beginning to emerge.

The victims are diverse: a mother buried in the same grave with her child, headmasters missing together with dozens of school students, illegal gold and diamond miners swept away by raging rivers and police officers washed away with their prisoners.

The Ministry of Information said 30 pupils, two headmasters and a teacher are missing.

A man arrives at a secondary school in Chimanimani, Zimbabwe, where families displaced by Cyclone Idai are sheltered. (Zinyange Auntony/AFP/Getty Images)

Zimbabwean President Emmerson Mnangagwa said Thursday that officers and prisoners were washed away.

In Mutare, fear gripped residents even though they are more than 140 kilometres from Chimanimani, the worst-hit part of Zimbabwe.

Maina Chisiriirwa, a city resident, said she buried her son-in-law, who had left the city to go to Chiadzwa diamond fields to mine illegally.

"There are no jobs and all he wanted was to feed his family. He was with his colleagues. They thought it would be easier to mine since the rains would keep the guards and the police away from patrolling," Chisiriirwa said. His colleagues survived but her son-in-law was swept away, she said.

A man who travelled several kilometres to a reception centre for survivors in Chimanimani said several of his colleagues were swept away as they tried to cross a river while fleeing from a mountain known for rich gold deposits and frequented by hordes of illegal miners.