Derna counts the cost of Libya flood disaster
Two dams burst there on Sunday afternoon after the storm hit, releasing a surge of water that tore through the city, sweeping away buildings and the people inside them.
By late Tuesday, the preliminary death toll from authorities in the politically fractured North African country was at least 2,300 dead.
Emergency services said more than 5,000 people were missing and about 7,000 were injured.
"The death toll is huge and might reach thousands," said Tamer Ramadan of the International Federation of Red Cross and Red Crescent Societies.
Media reports quoted a spokesman for the interior ministry of Libya's eastern government as saying "more than 5,200" people had died in Derna.
The city, 250 kilometres (150 miles) east of Benghazi, is ringed by hills and bisected by what is normally a dry riverbed in summer, but which became a raging torrent of mud-brown water that also swept away several major bridges.
Derna was home to about 100,000 people, and many of its multi-storey buildings on the banks of the riverbed collapsed, with people, their homes and cars vanishing in the raging waters.
With global concern about the disaster spreading, several nations offered urgent aid and rescue teams to help the war-scarred country that has been overwhelmed by what one UN official called "a calamity of epic proportions".
Elsewhere in Libya's east, aid group the Norwegian Refugee Council said on Tuesday "entire villages have been overwhelmed by the floods and the death toll continues to rise".
"Communities across Libya have endured years of conflict, poverty and displacement. The latest disaster will exacerbate the situation for these people. Hospitals and shelters will be overstretched."
Oil-rich Libya is still recovering from years of war and chaos that followed the 2011 NATO-backed popular uprising which toppled and killed longtime dictator Moamer Kadhafi.
The country is divided between two rival governments -- the UN-brokered, internationally recognised administration based in Tripoli, and a separate administration in the disaster-hit east.
Rescue teams from Turkey have arrived in eastern Libya, according to authorities. The United Nations and several countries offered to send aid, among them Algeria, Egypt, France, Italy, Qatar and Tunisia.
France is sending a field hospital and around 50 military and civilian personnel able to treat 500 people a day, Paris said on Tuesday.