Rescue teams combed through the rubble of an oceanfront apartment block near Miami Beach that partially collapsed Thursday, killing at least one person and leaving another 99 unaccounted for amid fears of a much higher death toll.

An unknown number of residents are feared to have been asleep in the 12-story building, in the town of Surfside, when the collapse in the wee hours of Thursday morning reduced a large portion of it to a pile of debris, exposing the interiors of gutted apartments.

"One side of the building just fell completely. It doesn't exist anymore," said Nicolas Fernandez, 29, an Argentinian resident of Miami who had yet to hear from friends who were staying overnight in his family's unit in the building. 

"I don't know about them. I don't know if they are alive," he told AFP.

Authorities said they were still without news of 99 people who may have been inside the building when part of it came crashing down. 

So far 102 others have been accounted for, Miami-Dade Mayor Daniella Levine Cava said.

Search and rescue teams with sniffer dogs will work through the night, Levine Cava said. "As the day comes to an end, their day does not," she said at an evening news conference, the last until Friday morning.

At a Surfside community center, relatives of the missing cried as they waited for news. Tenants of the ruined building who were lucky enough to have been away when disaster struck pondered sudden homelessness.

Erick de Moura, 40, happened to spend Wednesday night at his girlfriend's house.

"I just came back and the scene is shocking," he said. "There is a lot of pain. I'm blessed that I am alive."

Around 55 apartments were affected by the collapse, according to Miami-Dade Fire Rescue Assistant Chief Ray Jadallah who told a news conference that emergency services arrived at the scene at around 1:30 am, evacuating 35 people from the building. 

The building was occupied by a mix of full-time and seasonal residents and renters, and officials have stressed it is unclear how many people were actually inside at the time.

"It's hard to get a count on it," Miami-Dade County Commissioner Sally Heyman told CNN. "You don't know between vacations or anything else," she said. "The hope is still there, but it's waning."

- 'Bracing for bad news' -
Some residents were able to walk down the stairs to safety while others had to be rescued from their balconies.

One death was confirmed by Surfside mayor Charles Burkett, and Heyman said some 14 survivors had been recovered from the rubble.

"It's a really, really tragic situation so we'll hope for the best in terms of additional recoveries, but we are bracing for some bad news just given the destruction that we're seeing," Governor Rob DeSantis told a news conference.

As hope receded of finding more survivors, the focus was on the recovery of possible victims amid the rubble, in a massive operation assisted by drones and dogs and involving dozens of police and firefighter units.

"Apparently when the building came down it pancaked, so there's just not a lot of voids that they're finding or seeing from the outside," Burkett said on NBC's Today show.

After speaking with mayor Levine Cava, President Joe Biden told reporters his administrationstood ready to send emergency resources to Florida "immediately" if requested.

"I say to the people of Florida, whatever help you want, that the federal government can provide, just ask us, we'll be there," he said.

- 'Like a bomb went off' -
Surfside's mayor said the reasons for the collapse were still unclear.

"It looks like a bomb went off, but we're pretty sure a bomb didn't go off, so it's something else," Burkett said.

Fernandez, the Argentinian resident of Miami, said that when his mother called him in the early hours to say the building had collapsed, he thought it was a joke -- and hung up.

"She calls me again and tells me: 'Nico, you know I would never joke about this. I need you to go over there.' We came running."

One witness, 25-year-old Julian Targowski, described the sound of the collapse.

"It was like a very bass-y, like boom boom, boom boom, that kind of thing," he told local television WFOR.

"Like, a ton of bass on a subwoofer, basically, like just two of them," he said. "Then my friend texted me that a building had exploded."

Local media said records showed the block was built in 1981 and had more than 130 units inside.

Heyman told CNN the building had been undergoing construction work on its roof, although she also stressed the reasons for the collapse were not clear. 

Families wait for news of survivors

Staring into space or sobbing, family members of those missing in the Florida apartment building collapse waited in anguish for news of loved ones, while others scrambled for a place to live after their home was destroyed.

The disaster in Surfside, near Miami Beach, left one person dead and 99 unaccounted for after the collapse early Thursday, with fears the toll may rise much higher.

"Everything is gone," said Erick de Moura, 40, who was at the town's community center trying to find temporary accommodation.

The Brazilian had lived for three years on the 10th floor of the oceanfront Champlain Towers South, part of which pancaked in the middle of the night for reasons yet unknown.

He survived because he had decided to stay at his girlfriend's house last night.

"I am homeless now. I lost my papers, documents, everything. My green card, my money," de Moura said.

"I just came back and the scene is shocking," he added. "There is a lot of pain. I'm blessed that I am alive."

As for his neighbors, "I think they're gone."

These residents had had the best view, overlooking the beach and ocean, until their part of the tower collapsed like a house of cards in the night.

Firefighters, police and search dogs spent the day trying to locate survivors in the rubble. A crane cleared away pieces of debris, while a helicopter patrolled from above.

The rest of the 12-story building was still standing, but all residents were evacuated and several surrounding streets cordoned off. 

The survivors and families of those missing gathered a few blocks away at the community center, where volunteers were providing water, coffee, lunches, and clothes, as well as emotional support.

Displaced residents were trying to figure out where they can now live, whether it's with relatives or in hotels which have offered rooms. Others waited grimly for news of their loved ones.

People come and go. Some are seated and motionless, while others are sobbing.

More than a thousand people have passed through the community center, said Ron Ben Hayoun, a 22-year-old volunteer and north Miami resident who had returned to his childhood neighborhood to help.

In this city with a large Jewish community, "we try to help everyone, we are not exclusive."

"It's pretty shocking. people are very emotional," said the young man, who had recently returned from Israel where he did his military service.