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 about 100,000 people, where multi-storey buildings on the river banks collapsed and houses and cars vanished in the raging waters.

Emergency services reported an initial death toll of more than 2,300 in Derna alone and said over 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) east 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 emergency service roughly matched the grim estimates provided by the Red Cross and by authorities in the east, who have warned the death toll may yet rise further.

"The death toll is huge and might reach thousands," said Tamer Ramadan of the International Federation of Red Cross and Red Crescent Societies, three of whose volunteers were also reported dead.

"We confirm from our independent sources of information that the number of missing people is hitting 10,000 persons so far," Ramadan added.

Elsewhere in Libya's east, aid group the Norwegian Refugee Council said "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."

- 'Catastrophic' situation -

A spokesman for the interior ministry of Libya's eastern government, quoted in media reports, said "more than 5,200" people had died in Derna.

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 village to the southeast.

More than 300 victims were buried Monday -- but vastly greater numbers were feared lost in the river that empties into the Mediterranean.

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 Oil Corporation, 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.

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 divided between two rival governments -- the UN-brokered, internationally recognised administration based in Tripoli, and a separate administration in the disaster-hit east.

Access to the east 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.

A Derna city council official described the situation as "catastrophic" and asked for a "national and international intervention".

- Aid offers -

Libya's UN-backed government under Abdelhamid Dbeibah announced three days of national mourning on Monday and emphasised "the unity of all Libyans".

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, and technicians to restore power.

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 and Tunisia.

"Jill and I send our deepest condolences to all the families who have lost loved ones in the devastating floods in Libya," US President Joe Biden said in a White House statement, adding his government was sending funds and additional support.

Russian President Vladimir Putin also offered his condolences.

"We share the pain and grief of the friendly people of Libya. And of course we are ready to provide the necessary assistance," he said in a statement from the Kremlin.

UN Secretary-General Antonio Guterres expressed "his heartfelt condolences to the Libyan authorities and families of those who have lost their lives as a result of Storm Daniel", his spokesman said.

burs/dv