The two rival governments in Libya are co-ordinating relief efforts for flood victims, the UN has said.
More than 5,300 people died after two dam bursts brought devastating floods to the eastern city of Derna.
Derna's mayor, Abdulmenam al-Ghaithi said that number could reach as high as 20,000, based on the extent of the area of the city that was destroyed.
Another official from Tobruk-based eastern government said the sea was "constantly dumping dozens of bodies".
"The estimated the number of deaths in the city could reach between 18,000 to 20,000, based on the number of buildings in the districts destroyed by the flood," Mr Ghaithi told the Saudi-owned television station Al Arabiya.
"We actually need teams specialised in recovering bodies. I fear that the city will be infected with an epidemic due to the large number of bodies under the rubble and in the water," Mr Ghaithi added.
At least 10,000 people are missing and tens of thousands more have been displaced.
Hichem Abu Chkiouat, minister of civil aviation in the administration that runs eastern Libya, told Reuters that the number of casualties could increase significantly as the "sea is constantly dumping dozens of bodies".
"The fear is that the total number will be even higher because there's so many people who are unaccounted for, they have not contacted their relatives, their phones are dead and we don't know if they are still under the rubble or if they are dead or if they are in the sea," said Guma El-Gamaty, a Libyan academic and head of the Taghyeer Party.
Officials have asked for more international help.
A UN official said that both the eastern and western governments had requested international aid and were talking to each other.
"Both governments have reached out to the international community requesting services and help," Tauhid Pasha, of the International Organisation for Migration, told BBC Radio 4's the World Tonight programme.
"The Government of National Unity [western government] has extended its support to us and its request on behalf of the entire country and they are also co-ordinating with the government in the east," he said.
"The challenge now is the international community responding accordingly to the needs and the requests of the governments," he added.
Mr Pasha said support needed to be scaled up "very, very quickly and to do so we need money" for the city, whose pre-flood population was about 90,000.
The Libyan state began to fracture after the fall of long-serving ruler Col Muammar Gaddafi in 2011.
It split between two rival administrations based in the capital Tripoli in the west and Tobruk in the east.
The country remains mired in conflict between numerous militias.
Prime Minister Abdul Hamid Dbeibah leads the UN-backed Government of National Unity in Tripoli.
Osama Hamad, the prime minister in the east, leads the rival House of Representatives.
However, many feel power there is really held by Gen Khalifa Haftar, who leads the Libyan National Army,
Gen Haftar received an Egyptian military delegation which came to offer aid and support after the disaster.
UN High Commissioner on Human Rights Volker Turk stressed that all political groups had to work together in the aftermath of the floods.
"This is a time for unity of purpose," he said.
"All those affected must receive support without regard for any affiliations. It is important that particular care is taken to ensure protection of groups in vulnerable situations who are rendered even more at risk in the aftermath of such a disaster."
Abdulkader Assad, political editor of the Libya Observer, said having an internationally recognised government in the west rivalled by another government in the east had hindered rescue efforts.
"We all know that Libya has been split between two governments for the last decade at least and we haven't actually felt the impact of this division because the presence of two governments was all about vying for power and taking control of the country and parts of the country," he said.
"But now that some of the cities are experiencing this natural disaster, this calamity, we could see that the lack of a unified centralised government is actually affecting the lives of people."
Libyan rescue teams searching for survivors in Derna are being supported on the ground by:
Search and rescue teams from Egypt and Tunisia
More than 160 personnel from Turkey
Firefighters from Italy and Spain
Tommaso Della Longa, spokesman for the president of the International Federation of Red Cross and Red Crescent Societies, said time to find survivors was running out.
"We know that unfortunately this window will close in the next hours but hope is still there," he said.
Libyan Red Crescent teams in the field, he said, described the disaster as like "a bombardment and earthquake... happening at the same time".
"What they are telling us are really stories of entire areas of the city are not any more there, villages that are completely destroyed and thousands of families that at the moment really need everything," Mr Della Longa added.
Usama Al Husadi, 52, has been searching for his wife and five children since the catastrophic flood hit.
"I went by foot searching for them... I went to all hospitals and schools but no luck," he told Reuters news agency as he wept with his head in his hands.
"We lost at least 50 members from my father's family, between missing and dead," he said.
The bodies of more than 80 Egyptian migrants killed in the flood were returned to Egypt, the country's emigration ministry said, and were buried in their respective towns.