Residents of Mayotte have spoken of "apocalyptic scenes" caused by the worst storm in 90 years to hit the French Indian Ocean region.

Cyclone Chido brought wind speeds of over 225km/h (140mph), flattening areas where the poorest lived in rooftop shacks.

"We haven't had water for three days now," said one resident of the capital, Mamoudzou. "Some of my neighbors are hungry and thirsty," said another.

Rescuers, including reinforcements from France, are combing through the rubble in search of survivors. It has been confirmed that twenty people are dead, but the chief executive of the area said that it may be thousands.

Authorities said they had difficulty establishing the number of deaths due to the large number of undocumented migrants - more than 100,000 - in a population of 320,000.

Widespread damage to infrastructure - with downed power lines and impassable roads - is severely hampering emergency operations.

The first flight with supplies has arrived with tarpaulins for emergency shelters but there is a severe shortage of food, water and shelter in some areas.

Mamoudzou resident John Balloz said he was surprised he did not die when the cyclone hit.

"I was screaming because I could see the end coming," he said.

"Everything is damaged, almost everything, the water treatment plant, electricity pylons, there is a lot to do.

"He can't do much, people are still holding, they haven't moved, they are waiting for help, help for food, to restore the electricity, and also water, there is no running water .”

Mohamed Ishmael, who also lives in the capital, He told the Reuters news agency that there was a "tragedy" and added: "You feel like you are after a nuclear war...

"Hunger is what worries me the most," Mayotte Senator Salama Ramia told French media. "There are people who have had nothing to eat or drink" since Saturday, she said.

Francois-Xavier Bieuville, the island's chief executive, told local media that the death toll could rise significantly once the damage was fully assessed. He warned that it would "definitely be several hundred" and reach thousands.

Mayotte's poor communities, including undocumented migrants who have traveled to French territory in an attempt to claim asylum, are thought to be particularly hard hit due to the vulnerable nature of their housing .

The Muslim tradition of burying the dead within 24 hours also meant that it was more difficult to record the number of people who died, the prefect said.

In addition to financial aid, 110 French soldiers have arrived to help with the rescue, with another 160 on the way.

After arriving in Mayotte, French Interior Minister Bruno Retailleau said it would take "days and days" to determine the loss of life.

The relief work is being coordinated from Reunion - another French overseas territory.

French Red Cross spokesman Eric Sam Vah told the BBC the situation was "chaotic".

He said they were only able to reach 20 out of 200 Red Cross volunteers in Mayotte and raised fears about the total number of deaths.

"All the slums have been completely destroyed, we have not received any reports of displaced people, so the reality could be dire in the coming days," Mr Vah told the Today program on BBC Radio 4.

France colonized Mayotte in 1841 - and before the beginning of the 20th century he added the three main islands that make up the Comoros Islands to his overseas territories.

The Comoros voted to become independent in 1974 but Mayotte decided to remain part of France.

The island's population is heavily dependent on French financial aid and has long struggled with poverty, unemployment and political instability.

Around 75% of the population lives below the national poverty line and unemployment is rising at around one in three.

Cyclone Chido also made landfall in Mozambique, where it brought flash floods, uprooting trees and damaging buildings about 25 miles (40km) south of the northern city of Pemba. Three deaths were reported.

The cyclone caused structural damage and power outages in the northern coastal areas of Nampula and Cabo Delgado on Saturday morning, local authorities said.

Guy Taylor, a spokesman for the aid agency Unicef ​​in Mozambique, said, "we were hit hard in the early hours of this morning".

"Many houses have been destroyed or badly damaged, and health care facilities and schools are out of order," he said.

Mr Taylor said Unicef ​​was concerned about "loss of access to critical services", including medical treatment, clean water and sanitation, and also "the spread of diseases such as cholera and malaria".

Chido is the last deadly storm to produce such high intensity.

It strengthened as a result of its long journey across the ocean, says Sarah Keith-Lucas from the BBC Weather Centre. The cyclone would have weakened if it had made landfall on Madagascar's rugged terrain.

But it is also true that climate change is having an impact - not necessarily in the frequency of storms but in strength, Keith-Lucas says.

The storm has now been downgraded to a "depression" and is expected to pass over southern Malawi, then the Tete region of Mozambique, before heading towards Zimbabwe overnight into Tuesday.

It could still bring 150-300mm of rain by the end of Tuesday.