A desperate search for thousands of people missing after catastrophic flooding in the Libyan city of Derna entered its third day on Wednesday, with thousands already confirmed dead and the toll expected to rise.

The flood was caused by a powerful storm that burst dams nearby, unleashing a torrent of water that has devastated a quarter or more of the Mediterranean city, washing away buildings along with their residents.

Officials in eastern Libya say the death toll so far stands at more than 5,000. A hospital director in the city told Reuters on Monday 1,700 bodies had been counted at his hospital, and that 500 more had been buried in another part of the city.

Some 10,000 people are estimated to be missing. Many are believed to have been swept out to sea.

Reuters journalists who visited Derna on Tuesday saw many bodies laid out on the ground in hospital corridors and people seeking to identify missing relatives as more dead were brought in.

One Derna resident, Mustafa Salem, told Reuters he had so far lost 30 members of his family.

Aid convoys and trucks carrying bulldozers were headed towards the city on Wednesday.

The flood unleashed enormous destruction, flipping and mangling cars and leaving Derna's streets covered in rubble, mud and debris.

Satellite photographs of the city from before and after the disaster show that what had been a relatively narrow waterway through the city center was now several times wider, with all the buildings that had run along it gone.

Extensive damage, with buildings missing, is also clearly visible in other parts of the city where flood waters broke out from the waterway.

Rescue operations are complicated by Libya being politically fractured.

The internationally recognized Government of National Unity is based in Tripoli, in the west. Derna is in an eastern area where a parallel administration operates, and where control is wielded by commander Khalifa Haftar's Libyan National Army.

Libya's Prime Minister Abdulhamid al-Dbeibah, head of the Tripoli-based government, said on Tuesday the floods were an unprecedented catastrophe. Libya's Presidential Council head Mohammed al-Menfi has called for national unity.

The UN Office for the Coordination of Humanitarian Affairs said emergency response teams had been mobilized to help on the ground. Governments including Qatar and Turkey have rushed aid to Libya. (Reuters-Yonhap)