Emergency workers are searching for survivors on Mayotte after it was struck by Cyclone Chido, with officials warning that islanders are "starving" as food and water runs out.
Winds reached above 124mph on the archipelago - France's poorest region, located nearly 5,000 miles from Paris in the Indian Ocean - during the cyclone that struck over the weekend.
Authorities have warned it could take days to count the number of people to have died, with local TV station Mayotte la 1ere reporting at least 20 people have died.
But many more are feared dead, with Mayotte Prefect Francois-Xavier Bieuville telling local TV on Sunday: "I think there are some several hundred dead, maybe we'll get close to a thousand, even thousands... given the violence of this event."
However, he said it was currently "extremely difficult" to get an exact number.
Population 'starving'
Many residents have also been left without power, and communications have been down in large parts of the island.
Authorities are also becoming concerned about a shortage of drinking water, with Mayotte senator Salama Ramia telling Sky News's Gillian Joseph on The World programme that more aid is needed.
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"The situation has been very hard," she said. "It's midnight, we have no lights, no water, no communication, and since yesterday, no food."
"The population is starving, I will say, and suffering," she added. "As we don't have any communication, we don't even know what is going on (on the other side of the island)."
Ms Ramia then said: "It is terrifying, but when you wake up and you see all around your house what it looks like, but you're still alive... you just thank God to be safe."
The French Red Cross described the devastation as "unimaginable" and said more than 20 tons of supplies - including drinking water, hygiene kits and buckets to boil water - were being shipped in from nearby Reunion.
It added it was impossible to give an exact number of victims. Genevieve Darrieussecq, the French health minister, also said any figures were likely to be major underestimates "compared to the scale of the disaster".
With homes demolished and debris strewn by the cyclone, France scrambled ships and army aircraft to rush rescue workers and aid to the territory.
Authorities have used military-style vehicles to clear trees from roads so rescuers and supplies could reach those in need.
But some areas of the island have been left inaccessible because of the damage. Mayotte's main hospital, as well as its only airport, also suffered extensive damage.
Residents have also spoken about the scale of the damage, with Camille Cozon Abdourazak telling Reuters: "It really is a war landscape.
"I don't recognise anything anymore. There's not even a tree left, the hills, there's not a blade of grass, it's extraordinary."
Another resident, Abdoulhamidi, said: "Nobody believed it would be that big.
"Those who live in bangas (makeshift homes) stayed in despite the cyclone, fearing their homes would be looted."
What are cyclones, hurricanes and typhoons?
All three storms are powerful tropical cyclones which are given a different name depending on where they are in the world.
Meteorologists use the term tropical cyclone to describe a rotating, organised system of clouds and thunderstorms that originate over tropical or subtropical waters and have closed, low-level circulation and are fed by warm air.
The storm is referred to as either a cyclone, a hurricane or a typhoon once it reaches maximum sustained winds of 74mph or higher.
The term cyclone is used when the storms occur in the South Pacific and Indian Ocean.
They are referred to as typhoons when they appear in the Northwest Pacific.
The word hurricane is used when the storms appear in the North Atlantic, central North Pacific and eastern North Pacific.
It comes as French President Emmanuel Macron held an emergency meeting on Mayotte in Paris, declaring a national day of mourning over the cyclone.
He said he plans to visit the overseas territory in the coming days "to support our fellow citizens, civil servants and the emergency services".
Three dead in Mozambique
Already named the worst cyclone to hit Mayotte in nearly a century by weather service Meteo France, Chido also struck the nearby islands of Comoros and Madagascar on Saturday.
The cyclone then continued west and made landfall in Mozambique on Sunday.
While it quickly weakened and was reclassified as a tropical storm, local news outlets report at least three people died in the north of the country.
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Several homes in Mozambique were also destroyed, as aid agencies warn more than two million people could be affected.
The European Parliament observed a minute of silence for the victims - before the chamber's president Roberta Metsola vowed support for the region.
"Mayotte is Europe, and Europe will not abandon you," she said.