A powerful storm burst the dams on Sunday night, with water hurtling down a seasonal riverbed that bisects the eastern Libyan city of Derna, washing multi-storey buildings into the sea with sleeping families inside.

Confirmed death tolls given by officials have varied but all are in the thousands, with thousands more on lists of the missing.

Derna mayor Abdulmenam al-Ghaithi said deaths in the city could already reach 18,000-20,000, based on the extent of the damage.

He told Reuters he was afraid the city would now be infected with an epidemic “due to the large number of bodies under the rubble and in the water”.

The World Meteorological Organisation said the huge loss of life could have been avoided if Libya – a failed state for more than a decade – had a functioning weather agency in place.

“If there would have been a normally operating meteorological service, they could have issued warnings,” WMO Secretary-General Petteri Taalashe said in Geneva.

“The emergency management authorities would have been able to carry out evacuations… And we could have avoided most of the human casualties.”

Other commentators drew attention to warnings given in advance, including an academic paper published last year by a hydrologist outlining the city’s vulnerability to floods and the urgent need to maintain the dams that protected it.

Mohamed al-Menfi, head of the three-member council that acts as the presidency in Libya’s internationally recognised government, said on X that the council had asked the attorney general to investigate the disaster.

Those whose actions or failure to act were responsible for the failure of the dam should be held accountable, along with anyone who held up aid, he said.

Rescue teams arrived from Egypt, Tunisia, the United Arab Emirates, Turkey and Qatar.

Among countries sending aid, Turkey sent a ship carrying equipment to set up two field hospitals.

Italy sent three planes of supplies and personnel, as well as two navy ships that had difficulty offloading because Derna’s debris-choked port was almost unusable.

The World Health Organisation said it would release $US2 million ($A3.1 million) from its emergency fund to support the victims, calling the floods a “calamity of epic proportions”.

It added it would send trauma, surgical and emergency supplies from its logistics hub in Dubai.

Rescue work is hindered by the political fractures in a country of seven million people, at war on-and-off and with no central government since a NATO-backed uprising toppled Muammar Gaddafi in 2011.

An internationally recognised Government of National Unity  is based in Tripoli, in the west.

A parallel administration operates in the east, under control of the Libyan National Army of Khalifa Haftar, who failed to capture Tripoli in a bloody 14-month siege that unravelled in 2020.

Derna has been particularly chaotic, run by a succession of armed Islamist groups, including at one point Islamic State, before being uneasily brought under Haftar’s control.

A delegation of GNU ministers were expected in Benghazi in the east on Thursday to show solidarity and discuss relief efforts, a rare occurrence since the eastern-based parliament rejected their administration last year.

Viewed from high points above Derna, the once densely populated city centre is now a wide, flat crescent of earth with stretches of mud.

Nothing but rubble and a washed out road were left on Thursday at the site of the dam that had once protected the city.

The desert riverbed, or wadi, had already subsided back to a trickle.

Below, the beach was littered with clothes, toys, furniture, shoes and other possessions swept out of homes by the torrent.

Streets were covered in deep mud and strewn with uprooted trees and hundreds of wrecked cars, many flipped on their sides or roofs.

One car was wedged on a gutted building’s second-floor balcony.

-with AAP