Emergency workers sifting through the mud and rubble of the flood disaster in Libya are still hopeful of finding survivors, the Red Cross chief in the country said Friday.

"The hope is there, is always there, to find people alive," said Tamer Ramadan, the head of the International Red Cross and Red Crescent's rescue effort in the North African country.

At least 4,000 people are thought to have perished but provisional figures vary enormously.

Ramadan did not give an estimate of the toll, saying the "the definite numbers will be available upon the finalisation of the assessment of the ground."

The UN aid chief meanwhile said the extent of the catastrophe was unclear as Libya is divided between two rival authorities.

"I think the issue for us in Libya is of course coordinating with... the government and then the other authority in the east of the country," Martin Griffiths, Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator said, adding: "We don't know the extent of the problem."

Calling the situation "catastrophic", the United Nations launched an appeal for more than $71 million to respond to the "most urgent needs of 250,000 people targeted out of the 884,000 people estimated to be in need".

Griffiths welcomed a proposal by the mayor of the port city of Derna, the site of a tsunami-sized flash flood after two dams burst, for rescuers and aid to be sent there by sea.

"You've got obviously maritime access, you would ship in aid as well," he said. "You don't choose one route over another, you do all of them. You still keep coming in from the land... but certainly, adding the maritime option makes complete sense".