The floods were caused by torrential rains from Storm Daniel, which made landfall in Libya on Sunday after earlier lashing Greece, Bulgaria and Turkey.
At least 2,300 people were killed in Libya and thousands more were reported missing after catastrophic flash floods broke river dams and tore though an eastern coastal city, devastating entire neighbourhoods.
As global concern spread, multiple nations offered to urgently send aid and rescue teams to help the war-scarred country that has been overwhelmed by what one UN official labelled "a calamity of epic proportions".
Massive destruction shattered the Mediterranean coastal city of Derna, home to 100,000 people, where multi-storey buildings on the riverbanks collapsed and houses and cars vanished in the raging waters.
Libyan emergency services on the ground reported an initial death toll of more than 2,300 in Derna alone and said more than 5,000 people remained missing while about 7,000 were injured.
"The situation in Derna is shocking and very dramatic," said Osama Ali of the Tripoli-based Rescue and Emergency Service. "We need more support to save lives because there are people still under the rubble and every minute counts."
The floods were caused by torrential rains from Storm Daniel, which made landfall in Libya on Sunday after earlier lashing Greece, Bulgaria and Turkey.
Derna, 250 kilometres (150 miles) west of Benghazi, is ringed by hills and bisected by what is normally a dry riverbed in summer, but which has turned into a raging torrent of mud-brown water that also swept away several major bridges.
The number of dead given by the Libyan service roughly matched the grim early estimates provided by the Red Cross and by local authorities in the eastern region, who have warned the death toll may yet rise further.
"The death toll is huge and might reach thousands," Tamer Ramadan of the International Federation of Red Cross and Red Crescent Societies earlier told reporters.
"We confirm from our independent sources of information that the number of missing people is hitting 10,000 persons so far," Ramadan said via video link from Tunisia, which borders Libya.
Footage on Libyan TV showed dozens of bodies, wrapped in blankets or sheets, on Derna's main square, awaiting identification and burial, and more bodies in Martouba, a village about 30 kilometres to the southeast.
More than 300 victims were buried Monday, many in mass graves -- but vastly greater numbers of people were feared lost in the waters of the river that empties into the Mediterranean Sea.
Oil-rich Libya is still recovering from the years of war and chaos that followed the 2011 Nato-backed popular uprising which toppled and killed longtime dictator Moamer Kadhafi.
The country is now divided between two rival governments -- the UN-brokered, internationally recognised administration based in the capital Tripoli in the west, and a separate administration in the eastern region impacted by the flood disaster.
Access to the eastern region is limited. Phone and online links have been largely severed, but the administration's prime minister Oussama Hamad has reported "more than 2,000 dead and thousands missing" in Derna alone.
The military strongman backing the eastern administration, Khalifa Haftar, issued a similar warning.
A Derna city council official described the situation as "catastrophic" and asked for a "national and international intervention", speaking to TV channel Libya al-Ahrar.
Rescue teams from Turkey have arrived in eastern Libya, according to authorities, and the UN and several countries offered to send aid, among them Algeria, Egypt, France, Italy, Qatar, Tunisia and the United States.
The storm also hit Benghazi and the hill district of Jabal al-Akhdar. Flooding, mudslides and other major damage were reported from the wider region, with images showing overturned cars and trucks.
Libya's National Petroleum Company, which has its main fields and terminals in eastern Libya, declared "a state of maximum alert" and suspended flights between production sites where it said activity was drastically reduced.
Libya's UN-brokered government under Abdelhamid Dbeibah announced three days of national mourning on Monday and emphasised "the unity of all Libyans" in the face of the disaster.
Aid convoys from Tripoli were heading east and Dbeibah's government announced the dispatch of two ambulance planes and a helicopter, as well as rescue teams, canine search squads and 87 doctors, as well as technicians to restore power.
Italy's Foreign Minister Antonio Tajani wrote that Rome was "responding immediately to requests for support for the floods... an assessment team is already on its way, coordinated by our civil protection unit".
Concern has spread, with many foreign leaders voicing their condolences.
The United States embassy said it had "issued an official declaration of humanitarian need in response to the devastating floods in Libya ... We are coordinating with UN partners and Libyan authorities to assess how best to target official US assistance".
European Council president Charles Michel, writing on X, formerly Twitter, noted the "harrowing images from Libya following deaths & destruction caused by floods, especially in the east... EU stands ready to help those affected by this calamity."