(Reuters) - Libyan authorities demanded an investigation on Thursday into whether human failings were to blame for thousands of deaths in the worst natural disaster in the country's modern history, as survivors searched for loved ones washed away by floods.

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

Confirmed death tolls given by officials have varied. 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 Organization 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 evacuation of the people. 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.

Usama Al Husadi, a 52-year-old driver, had been searching for his wife and five children since the disaster.

"I went by foot searching for them ... I went to all hospitals and schools but no luck," he told Reuters, weeping with his head in his hands.

Husadi, who had been working the night of the storm, dialled his wife's phone number once again. It was switched off.

"All were swept away by the valley into the sea," he said. "May God have mercy upon them and grant them heaven."

"We lost at least 50 members from my father’s family, between missing and dead," he said.

Wali Eddin Mohamed Adam, 24, a Sudanese brick factory worker living on Derna's outskirts, had awakened to the boom of the water on the night of the storm and rushed to the city centre to find it was gone. Nine of his fellow workers were lost, and around 15 others had lost their families, he said.