The mayor of the eastern Libya port city of Derna estimates between 18,000 and 20,000 people have died in flooding
Abdulmenam Al-Ghaithi told al-Arabiya TV his estimate was based on the number of districts completely destroyed when two dams burst.
Many bodies remain under the rubble of collapsed buildings or in the sea.
More than 5,000 people are known to have died, and at least 10,000 are missing.
Streets were swept away in the torrents and bodies are being recovered from sea.
The two rival governments in Libya are co-ordinating relief efforts for flood victims.
Rescue teams have arrived from countries including Egypt, Tunisia, Italy, Spain and Turkey.
All that is left of much of Derna is the rubble of collapsed buildings and the bodies of people killed in the floods, a photographer based in the eastern city has told the BBC.
Taha Muftah told Newshour on the World Service that the sound of the dams collapsing was like an air strike or heavy gunfire. BBC News
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