UN agencies warned Monday that Libya's flood-stricken city of Derna, where thousands were killed a week ago, faces the threat of disease outbreaks that could bring "a second devastating crisis".

The massive flash flood that has killed over 3,000 people and left thousands more missing came as the war-scarred North African country was lashed by the hurricane-strength Storm Daniel on September 10.

Traumatised residents, 30,000 of whom are now homeless, badly need clean water, food and basic supplies amid a growing risk of cholera, diarrhoea, dehydration and malnutrition, UN agencies warn.

"Teams from nine UN agencies have been on the ground delivering aid and support to those affected by Storm Daniel and the flash flooding for the last few days," said the UN Support Mission in Libya.

But it warned that local officials, aid agencies and the UN World Health Organization "are concerned about the risk of disease outbreak, particularly from contaminated water and the lack of sanitation".

"The team continues to work to prevent diseases from taking hold and causing a second devastating crisis in the area," UNSMIL said in a statement.

Rapidly rising waters burst two upstream river dams in Derna, sending a late-night tidal wave crashing through the centre of the coastal city of 100,000 that swept entire residential blocks into the Mediterranean.

UN teams also including the children's relief fund UNICEF, refugee agency UNHCR and World Food Programme have been in and around Derna for the past few days to help the survivors.

UNICEF teams have delivered "medical kits to primary care services to support 15,000 people for three months" while the UNHCR distributed supplies including blankets, tarpaulins and kitchen equipment to 6,200 displaced families in Derna and Benghazi, said UNSMIL.

"So far food rations have been distributed to over 5,000 households through the World Food Programme, and 28 tonnes of medical supplies have been shipped by charter flight from the World Health Organization, which has also donated ambulances and medical kits."

Last week, the United Nations launched an aid appeal for more than $71 million for the emergency response in Derna and other parts of eastern Libya.