The UN’s World Heath Organisation said that up to 1.8 million had been affected by the torrential rains in Libya, where several other cities were also hit by the storm.
Its spokeswoman Margaret Harris added that it was an “epic” disaster and there had not “been a storm like this in the region in living memory”.
Meanwhile, aid convoys and trucks carrying bulldozers were travelling to Derna on Wednesday.
The flood unleashed enormous destruction, leaving Derna’s streets covered in rubble, mud and debris.
Satellite photographs of the city from before and after the disaster show that what had been a relatively narrow waterway through the city centre was now several times wider, with all the buildings that had run along it gone.
Satellite images released by Maxar Technology shows Libya’s eastern city of Derna before the flash floods on September 7 and (R) during the flash floods on September 12
Extensive damage, with buildings missing, is also clearly visible in other parts of the city where flood waters broke out from the waterway.
In response to the disaster, the British Red Cross said it had launched an appeal to help the stricken and that three Libyan Red Crescent volunteers had already lost their lives while trying to save others in what was “an extremely dangerous situation”.
Richard Blewitt, an executive director of the charity, said: “The floods in Libya have caused a shocking level of destruction, thousands are thought to have died and thousands more are still missing. The priority right now is search and rescue so we can save as many lives as possible.
“Red Cross teams are responding, providing first aid to those that need it, searching for the missing and helping to reunite families.
“However, the situation is extremely dangerous and sadly, three Libyan Red Crescent volunteers lost their lives while trying to save the lives of others. Our thoughts are with their families and all the families who have lost loved ones in these floods.”
As well as the danagerous conditions, rescue operations are further complicated by the political divisions in Libya, which has been in turmoil since the ousting of the dictator Col Gaddafi in 2011.
The internationally recognised Government of National Unity is based in Tripoli, in the west. Derna is in an eastern area where a parallel administration operates and control is wielded by commander Khalifa Haftar’s Libyan National Army.
Libya’s Prime Minister Abdulhamid al-Dbeibah, head of the Tripoli-based government, has described the floods as an unprecedented catastrophe. The head of Libya’s Presidential Council, Mohammed al-Menfi, has called for national unity.
The U.N. Office for the Coordination of Humanitarian Affairs said emergency response teams had been mobilised to help. Governments including Qatar and Turkey have already rushed aid to Libya.
