Aid teams have been battling to reach and help survivors and locate the dead after thousands were killed when two dams burst after intense rainfall from Storm Daniel on Sunday, with 23ft waves washing away whole neighbourhoods in the city.
It came after the death toll from the tragedy rose on Thursday to 11,300. The Libyan Red Crescent said another 10,100 were reported missing. Derna mayor Abdel-Moneim al-Ghaithi said the tally could reach 20,000, given the number of neighbourhoods that were washed away.
Victims of the floods that were washed out to sea have reportedly drifted on to beaches more than 100km away.
Thousands of people have been buried in mass graves outside Derna and nearby towns and cities.
The United Nations has said most of the deaths could have been avoided.
“If there would have been a normal operating meteorological service, they could have issued the warnings,” World Meteorological Organization head Petteri Taalas said. “The emergency management authorities would have been able to carry out the evacuation.”
Rescue teams were searching wrecked buildings in the city centre and divers were combing the sea off Derna.
Soon after the storm hit the city Sunday night, residents said they heard loud explosions when the dams outside the city collapsed. Floodwaters gushed down Wadi Derna, a valley that cuts through the city, crashing through buildings and washing people out to sea.
Lori Hieber Girardet, the head of the risk knowledge branch the U.N. Office for Disaster Risk Reduction, told The Associated Press on Thursday that because of years of chaos and conflict Libyan "government institutions are not functioning as they should."
As a result, she said, "The amount of attention that should be paid to disaster management, to disaster risk management isn't adequate."
The city of Derna is governed by Libya's eastern administration, which is backed by the powerful military commander Khalifa Hiftar.