Bodies were washing ashore in eastern Libya on Wednesday, swelling the death toll from a storm that swept whole neighbourhoods out to sea, with thousands already confirmed dead and many thousands more still missing.

The flood torrent, unleashed by a powerful storm on Sunday night that burst dams, obliterated around a quarter or the Mediterranean city of Derna. Whole multi-storey buildings were swept away with sleeping families inside.

The "sea is constantly dumping dozens of bodies", Hichem Abu Chkiouat, minister of civil aviation in the administration that runs eastern Libya, told Reuters by phone.

"We have counted more than 5,300 dead so far, and the number is likely to increase significantly and may even double because the number of missing people is also thousands," he added.

Tens of thousands of people had been made homeless, he said, appealing for international aid and adding that Libya did not have the experience to deal with the aftermath of such a disaster.

Officials say at least 10,000 people are feared missing or dead, though tolls confirmed dead so far vary. Tariq Kharaz, a spokesperson for the eastern authorities, said 3,200 bodies had been recovered, and 1,100 of them had yet to be identified.

The U.N. migration agency, the International Organization for Migration, said at least 30,000 people had been displaced in Derna.

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, all its building swept away.