LONDON — For Ehdaa Bujeldain, an English teacher living with her family in Bab-Tobruk, in the mountains of Derna, eastern Libya, it sounded like a bomb going off in the middle of the night.

"On Sunday night, at 3 a.m., me and my family heard something like an explosion," she tells NPR by phone. "We lost electricity and connection. We didn't know what had happened. Then we heard it was a dam in Derna that had collapsed."

Four days passed with no electricity or internet, and it is only in the past couple of days that she and her family have started to learn the full extent of devastation from this week's floods.

With each day that passes, they learn of new losses — colleagues, friends and family that were killed in the floods.

"Half of the city vanished. My mum's relatives, my friends, my coworkers. They are all dead," Bujeldain says.

Najib Tarhoni, a doctor working in the nearest large hospital to Derna, in the city of Benghazi, has family members who survived the flood. They made it to safety in Benghazi, he says, but are forever changed.

"These people," he says, "are ghosts in shells. They have seen death, not just in their families but within themselves as well. Their souls are crushed, their hope is lost. How can you come back from such a thing? It's close to annihilation."

Five days on, figures vary for the number of dead from the catastrophic flooding that hit Derna. Libya's Red Crescent organization estimated Thursday that 11,300 people have died. The city's mayor said the death toll could reach 20,000.

Aid teams are arriving in the city, but their efforts have often been thwarted by damaged infrastructure and lack of access to power, water and fuel.

United Nations aid chief Martin Griffiths called the scale of the flood "appalling" and said it was a "massive reminder" of the challenges posed by climate change.

Derna is a Mediterranean coastal city bisected by a seasonal river, the Wadi Derna, that flows south from the highlands. For those living by the Wadi Derna valley, there was barely any warning before the floodwaters swept in, tsunami-like, in the early hours of Monday morning.

Nasib Almnsori is from Derna and now lives in nearby Tobruk, just over 100 miles away. He lost three cousins and their families in the flood. Other cousins who survived have come to stay with him.

The rainfall didn't seem unusual at first, he says. Every year, the Wadi Derna valley fills up with rainwater, creating the seasonal river. Family and friends were sending him videos of the rainwater in the valley on Sunday evening, just as they always have in past years.

This time though, the amount of rain from Storm Daniel was extraordinary. And then, at around 2 a.m. on Monday, two dams collapsed.

"The water reached the second floor of my cousin Seraj's house," he says. "He looked outside and saw a lot of water going into the house. He woke his family up and told them to go up onto the third floor, onto the roof, that's how they survived."

Almnsori says people living in single- or two-story houses by the Wadi Derna couldn't escape as easily.

The morning after, his brother made a difficult journey to look for relatives, confronting roads cut off by the floods. One of their cousins, 37-year-old Khadija, was missing.

"The situation was a disaster," he says. "People [were] buried under their houses. First they didn't find her, they only found her husband and the children, so they just kept looking. They didn't find her in the house because the flood took her away."

Only her five-year-old son, Fares, had survived.

"I know families where every single one of them died. No one stayed alive. Compared to my tragedy, that's really huge," Almnsori says.

Taha Muftah, a photojournalist living on the west side of Derna, spent Friday morning walking around the center and taking stock of the devastation.

"The damage is huge, unfathomable. We are begging with the world to send help, to listen to our plea," he says.

Ibrahim Ozer, with the Turkish Red Crescent organization, was part of a search and rescue team in Derna earlier in the week. He described the difficulty of transporting and delivering aid across a city split in half.

"There is one river, it is connecting east to west, all the bridges collapsed, there is no passage from one side to the other side," he says. "It's not a regular flood — it's like a storm, a flood and an earthquake."

Tarhoni, the doctor in Benghazi, warns that the most difficult weeks lie ahead.

"One catastrophe is done and there is another to come," he says. "The thousands and thousands of people who [lost everything], who lost their houses, their jobs — these people now need jobs, they need taking care of, they need psychological support."