Residents of Mayotte have spoken of “apocalyptic scenes” caused by the worst storm in 90 years to hit the French Indian Ocean territory.
Cyclone Chido brought wind speeds of more than 225km/h (140mph), flattening areas where the poorest lived in sheet-metal roof shacks.
“We’ve had no water for three days now,” said one resident of the capital city Mamoudzou. “Some of my neighbours are hungry and thirsty,” another one said.
French President Emmanuel Macron said he will be travelling to Mayotte “in the coming days”, as he pledged to support fellow citizens, civil servants and emergency services involved in rescue efforts.
Rescue workers, including reinforcements from France, are combing through the debris searching for survivors. Twenty people have been confirmed dead, but the local prefect said it could be thousands.
Macron said he will declare a national day of mourning, in light of “this tragedy, which has shaken each and every one of us”.
Authorities said they were having difficulty establishing the number of deaths due to the large number of undocumented migrants – over 100,000 – in a population of 320,000.
Widespread damage to infrastructure – with downed power lines and impassable roads – is severely hindering emergency operations.
Supplies have begun to arrive, but there are severe shortages of food, water and shelter in certain areas. Some 85% of the territory remains without power, and about 20% of phones appear to be working. Some areas are beginning to get tap water.
But for Amalia Mazon, a 27-year-old midwife from Brussels who has been working at the island’s central hospital, access to drinking water and food continues to be a concern.
“The water here is completely yellow. It’s unusable for us,” Ms Mazon told the BBC.
“We feel completely abandoned, and we don’t even know if help is coming. We have no news, we have no idea,” the midwife added.
Acting French health minister Geneviève Darrieussecq said the healthcare system in the archipelago had been “degraded” by the cyclone.
France colonised Mayotte in 1841 – and by the turn of the 20th Century added the three main islands that constitute the Comoros archipelago to its 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 struggled with poverty, unemployment and political instability.
About 75% of the population live below the national poverty line and unemployment hovers at around one in three.
“The images are apocalyptic. It’s a disaster, there’s nothing left,” a nurse working at the main hospital in Mamoudzou told BFM TV.
Mamoudzou resident, John Balloz, said he was surprised he did not die when the cyclone struck.
“Everything is damaged, nearly everything, the water treatment plant, electric pylons, there’s a lot to do.”
Mohamed Ishmael, who also lives in the capital, told Reuters news agency: “You feel like you are in the aftermath of a nuclear war… I saw an entire neighbourhood disappear.”
“It’s the hunger that worries me 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 prefect, told local media the death toll could rise significantly once the damage was fully assessed. He warned it would “definitely be several hundred” and could reach the thousands.
Mayotte’s impoverished communities, including undocumented migrants who have travelled to the French territory in an effort to claim asylum, are thought to have been particularly hard hit due to the vulnerable nature of their housing.
The Muslim tradition of burying the dead within 24 hours also meant documenting the number of those who have perished was more difficult, the prefect said.
In addition to aid, 110 French soldiers have arrived to help with the rescue, with another 160 on the way. Some 800 others from the ranks of volunteers helping during emergencies were also being sent to join local police units.
After arriving in Mayotte, French Interior Minister Bruno Retailleau said “days and days” would be needed to ascertain human losses.
The relief operation is being co-ordinated from Reunion – another French overseas territory.
French Red Cross spokesman Eric Sam Vah told the BBC the situation was “chaotic”.
He said the organisation had been able to reach only 20 out of 200 Red Cross volunteers in Mayotte and echoed fears about the overall number of deaths.
“The totality of the slums have been totally destroyed, we haven’t received any reports of displaced people, so the reality could be terrible in the coming days,” the spokesman told BBC Radio 4’s Today programme.
Cyclone Chido also made landfall in Mozambique, where it brought flash flooding, uprooted trees and damaged buildings about 25 miles (40km) south of the northern city of Pemba. Three deaths have been reported.
The cyclone caused structural damage and power outages in the northern coastal provinces of Nampula and Cabo Delgado on Saturday morning, local authorities reported.
Guy Taylor, a spokesperson for aid agency Unicef in Mozambique, said “we were hit very hard in the early hours of this morning”.
“Many houses were destroyed or seriously damaged, and healthcare facilities and schools are out of action,” he added.
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 like cholera and malaria”.
Chido is the latest deadly storm to form of such high intensity.
It strengthened as a result of its long track over the ocean, says Sarah Keith-Lucas from the BBC Weather Centre. The cyclone would have weakened had it made landfall on Madagascar’s rugged terrain.
But it is also the case that climate change has an impact – not necessarily in the frequency of storms but in the strength, Keith-Lucas says.
The storm has been now downgraded to a “depression” and is due to cross southern Malawi, then Mozambique’s Tete province, before heading towards Zimbabwe overnight into Tuesday.
It may still bring 150-300mm of rain by the end of Tuesday.