DERNA, Libya - Residents and rescue workers in the devastated Libyan city of Derna are struggling to cope with the thousands of corpses washing up or decaying under rubble, after a flood that smashed down buildings and swept people out to sea.

The World Health Organization and other aid groups called on authorities in Libya to stop burying flood victims in mass graves.

They said these could bring long-term mental distress to families or might cause health risks if located near water.

Libya was hit by torrential rain from a Mediterranean storm on Sunday. The eastern city of Derna bore the brunt of the disaster.

Thousands were killed and thousands more are missing.

Since then, a UN report showed more than 1,000 people had so far been buried in mass graves.

"Bodies are littering the streets, washing back on shore and are buried under collapsed buildings and debris. In just two hours, one of my colleagues counted over 200 bodies on the beach near Derna," Mr Bilal Sablouh, the ICRC's regional forensics manager for Africa, told a briefing in Geneva.

Swathes of Derna were obliterated when storm water swept down a usually dry riverbed, smashed two dams above the city and brought down whole buildings while families were asleep.

The International Organization for Migration mission in Libya said on Friday that more than 5,000 people are presumed dead, with 3,922 deaths registered in hospitals, and over 38,640 were displaced in the flood-stricken north-eastern region.

The death toll could be far higher, officials say.

"We should be afraid of an epidemic," 60-year-old Nouri Mohamed said at a bakery, which has been offering loaves for free to help Derna's shattered community. "There are still bodies underground ... Now there are corpses starting to smell."

The UN health agency together with the International Committee of the Red Cross and International Federation of the Red Cross and Red Crescent Societies called for better managed burials.

"We urge authorities in communities touched by tragedy to not rush forward with mass burials or mass cremations," Mr Kazunobu Kojima, medical officer for biosafety and biosecurity in the WHO's Health Emergencies Programme, said in the statement.

The statement called for demarcated and documented individual graves, saying hasty interments could lead to mental anguish for families as well as social and legal problems.

The bodies of victims of trauma from natural disasters "almost never" pose a health threat, it said, unless they are in or near fresh water supplies since corpses may leak faeces.

Dealing with the dead

Thursday's UN report said more than 1,000 bodies in Derna and over 100 bodies in Al Bayda, another coastal city which was hit by flooding, had been buried in mass graves.

The ICRC sent a cargo flight to Benghazi, eastern Libya's largest city, on Friday with 5,000 body bags. Other aid has also been coming in from abroad.

The Norwegian Refugee Council, which has a team of 100 in Libya, said dead body management is the most pressing concern.

"I've heard from my team that there are mass graves where rescue workers were appealing: 'Don't bring us food, don’t bring us water, bring us body bags'," the NRC's Ahmed Bayram said.

Libya is a nation divided by a decade of conflict and political chaos. And some residents are frustrated that the country's fractured authorities are not acting faster to help.

"The state is no of use to us," said Mr Saad Rajab Mohamed al-Hasi, a 50-year-old security officer who lives in Susah, a town about 60km away that was also damaged by flooding. "Now I’m in the street with my children and wife."

UN aid chief Martin Griffiths told the Geneva briefing that Libya needs equipment to find people trapped in sludge and damaged buildings after the floods as well as primary health care to prevent a cholera outbreak among survivors.

"Priority areas are shelter, food, key primary medical care because of the worry of cholera, the worry of lack of clean water," Mr Griffiths said. REUTERS