The UN warned on Monday that disease outbreaks could bring "a second devastating crisis" to Libya a week after a huge flash flood shattered the coastal city of Derna, sweeping thousands to their deaths.

Local officials, aid agencies and the World Health Organisation "are concerned about the risk of disease outbreak, particularly from contaminated water and the lack of sanitation," the United Nations said.

The flash flood that has killed nearly 3,300 people and left thousands more missing came after the war-scarred North African country was lashed by the hurricane-strength Storm Daniel on September 10.

Tens of thousands of traumatised residents are homeless and badly in need of clean water, food and basic supplies amid a growing risk of cholera, diarrhoea, dehydration and malnutrition, UN agencies have warned.

Libya's disease control centre banned citizens in the disaster zone from drinking water from local mains, warning that it is "polluted."

Rescue teams from several European and Arab countries kept up the grim search for bodies in the mud-caked wasteland of smashed buildings, crushed cars and uprooted trees.

The waters submerged a densely populated six-square-kilometre area in Derna, damaging 1,500 buildings of which 891 were totally razed, according to a preliminary report released by the Tripoli government based on satellite images.

One bereaved Derna resident, Abdul Wahab al-Masouri, lamented what has become of his city.

"We grew up here, we were raised here... But we've come to hate this place, we've come to hate what it has become," he said.

"The buildings, the neighbourhood, the villagers, the sheikhs... the wadi has returned to the state it was 1,000 years ago. People live in caves, the city looks dead, barren, there is no life left."

Bulldozers cleared roads from caked mud, including at a mosque as a foul smell permeated the air and a woman prayed for the children and grandchildren killed by the flooding.

Amid the chaos, the true death toll remained unknown, with untold numbers swept into the sea. (AFP)