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.
Macron said he will declare a national day of mourning, in light of "this tragedy, which has shaken each and every one of us".
Cyclone Chido brought wind speeds of more than 225km/h (140mph) to the French island, flattening areas where the poorest lived in sheet-metal roof shacks.
Rescue workers, including reinforcements from France, are continuing to comb through the debris searching for survivors.
Twenty people have been confirmed dead, but the local prefect said it could be thousands.
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