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.
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 the sea.
The two rival governments in Libya are coordinating relief efforts for flood victims.
Rescue teams have arrived from countries including Egypt, Tunisia, Italy, Spain and Turkey.
Derna mayor raises fears of many thousands of deaths
As many as 20,000 people are feared to have died in floods in Libya, according to a local official.
Catastrophic floods deluged the east of the country on Sunday.
The mayor of the port city Derna told Saudi TV News station Al Arabiya he estimated 18,000 to 20,000 died when two dams burst, releasing a tsunami of water as people slept.
His figures are based on the number of communities destroyed by the flood water, he told the channel.
Unrecovered bodies remain under rubble or in the sea, increasing the risk of disease.
Meanwhile, rival governments in Libya have requested international aid.