The United Nations is urgently seeking more than $71 million to assist those most in need after deadly flash floods swept Libya over the weekend.

Hurricane-strength Storm Daniel slammed into Libya on September 10, killing at least 4,000 people, with thousands more still missing.

In a flash appeal Thursday, the United Nations humanitarian agency OCHA said it expects the toll to rise.

The city of Derna, one of the hardest-hit locations, was reduced to a wasteland after two upstream dams burst Sunday.

OCHA said estimates suggest 30 percent of the city may have disappeared and with most roads collapsed local authorities are calling for a sea corridor to be established for relief and evacuations.

The entire seaside town of Sousse meanwhile remains submerged.

Calling the situation "catastrophic", OCHA said its humanitarian partners need $71.4 million to respond to the "most urgent needs of 250,000 people targeted out of the 884,000 people estimated to be in need".

On Wednesday, UN OCHA head Martin Griffiths had announced an immediate emergency fund of $10 million.

"Entire neighbourhoods have been wiped off the map. Whole families, taken by surprise, were swept away in the deluge of water," he said in a statement.

"Getting lifesaving supplies to people, prevent a secondary health crisis, and swiftly restore some kind of normality must override any other concern at this difficult time for Libya."

The United States, European Union, Turkey, United Arab Emirates and several other countries have also already sent or pledged aid, while foreign rescue teams have been deployed to search for survivors and recover bodies.