Libyans are living through a “doomsday” as the ocean dumps “dozens of bodies” back onto shore, and streets are littered with body bags and rotting corpses.

As the death toll from Storm Daniel rose past 5300 early on Thursday (AEST), one medical director feared as many as 20,000 people may have perished in the north African nation.

The mounting toll has meant bodies are being buried by the hundreds in mass graves as they are collected.

The “sea is constantly dumping dozens of bodies”, said Hichem Abu Chkiouat, minister of civil aviation in the administration that runs eastern Libya.

“We have counted more than 5300 dead so far, and the number is likely to increase significantly and may even double because the number of missing people is also thousands.”

Families were asleep in their homes when the powerful storm unleashed on eastern Libya, causing two dams to burst and obliterate about one-quarter of the port city of Derna.

As residents slumbered, whole multistorey buildings and neighbourhoods were swept out to sea.

At least 30,000 people have been displaced in Derna, said the United Nations International Organisation for Migration.

Officials say at least 10,000 people are feared missing or dead. Some 3200 bodies have been recovered, and 1100 of them have yet to be identified.

Al-Bayda medical centre director Abdul Rahim Maziq estimated a death toll as high as 20,000, The Guardian reported.

Others may still be trapped alive or crushed under rubble, and there are calls for professional rescue teams to help extract potential survivors.

“People are breathing under buildings and we can’t get them out,” said Dr Najib Tarhoni, appealing for help.

Journalist Johr Ali told the BBC Libyans were living through a doomsday.

“People are hearing the cries of babies underground, they don’t know how to get to them,” he said.

“People are using shovels to get the bodies from underneath the ground, they are using their own hands.

“The situation is beyond catastrophic.”

Some surviving Libyans said they had been sounding the alarm about cracks in the dams, but “no-one listened”.

Mr Abu Chkiouat appealed for international aid, adding that Libya did not have the experience to deal with the aftermath of such a disaster.

The devastation was clear from high points above Derna, where the densely populated city centre, built along a seasonal riverbed, was now a wide, flat crescent of muddy water, gleaming in the sun, its buildings swept away.

At a hospital, scores of bodies wrapped in blankets were laid out on the floor in corridors or outside on the pavement, for residents to try to identify them.

Mustafa Salem said his entire family lived near the river valley opposite a mosque.

They were asleep when the flood hit, and no one has been found alive.

“We are all neighbours,” he told Reuters.

“We lost 30 people so far, 30 members of the same family.

“We haven’t found anyone.”

As Reuters was on the road to return to the city on Wednesday, aid convoys and trucks carrying bulldozers could be seen heading in.

Satellite photographs of the city from before and after the disaster showed what had been a narrow waterway through the city centre was now a wide scar, with all the buildings that had run along it gone.

Extensive damage, with buildings missing, was also clearly visible in other parts of the city.

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Rescue operations are complicated by deep political fractures in the country of seven million people that has yet to rebuild a strong central government since the NATO-backed uprising that toppled Muammar Gaddafi in 2011.

An internationally recognised Government of National Unity is based in Tripoli, in the west, while a parallel administration operates in the east, including Derna.

Libya’s Tripoli-based Prime Minister Abdulhamid al-Dbeibah called the floods an unprecedented catastrophe.

Libya’s Presidential Council head Mohammed al-Menfi has called for national unity.

The UN Office for the Co-ordination of Humanitarian Affairs said emergency response teams had been mobilised to help on the ground.

Governments including Qatar and Turkey have rushed aid to Libya.

The United Arab Emirates has sent two aid planes carrying 150 tonnes of urgent food, relief and medical supplies to eastern Libya, the UAE’s state news agency WAM reported.