Florida apartment building collapse: Miami rescue workers pick through rubble for survivors

At least one person is dead and 99 unaccounted for after a high-rise apartment block in Florida partially collapsed during Thursday's early hours, sparking a major emergency response.

Online videos showed a large portion of the 12-storey building in the town of Surfside – just north of Miami Beach – reduced to rubble, with the apartments' interiors exposed.

Miami Dade authorities declared a "Level 5" mass casualty event meaning statewide emergency resources are required, according to reports.

Miami-Dade Fire Rescue officials said more than 80 units responded to the collapse at the building around 2am local time. They said 55 apartment units collapsed and first responders rescued 35 people trapped inside.

Miami-Dade Police Department spokesperson Alvaro Zabaleta said that 99 people are still unaccounted for, updating the previous figure of 51 missing people.

Surfside's mayor Charles Burkett confirmed at least one person had died in hospital following the collapse.

"This morning I was told by the chief that we treated 10 people on the site, two of those 10 were transported to the hospital. One has passed away," he said.

It was unknown how many people were injured or were inside the building at the time of the collapse.

Burkett said the building collapse was "less likely than a lightning strike".

"It looks like a bomb went off but we are pretty sure a bomb didn't go off," he said. "You just don't see buildings like this fall down in America."

"There is no reason for this building to go down like this unless someone literally pulls out the supports from underneath, or they get washed out, or there is a sinkhole, or something like that because it just went down," Burkett said.

He said the part of the building that remained intact had been checked and cleared of residents but he was not optimistic of finding survivors in the collapsed part of the building.

"The problem is the building has literally pancaked … It's heartbreaking because it doesn't mean, to me, that we're going to be successful, as successful as we want to be, to find people alive."

He said there was construction happening on the roof but the equipment was not heavy enough to cause the partial collapse.

Firefighters could be seen pulling a boy from the rubble alive.

Nicholas Balboa told CNN he was outside walking his dog when he heard and felt the ground shake.

He saw a plume of smoke and saw the collapsed building.

After running home to drop off his dog he came back to the site.

"I was thinking to myself, 'How could anyone survive?'" Balboa told the network.

"As I was thinking that, I heard a voice yelling."

He said that as he got closer, he saw the boy "sticking his hand up through the debris and I could see his hand and fingers wiggling".

"I began to climb the debris to try and get to him. I used the light on my phone to signal firefighters to come over. A police officer came over and he radioed for fire rescue to come over," he added. "So then they began their efforts to get him out. It was him and his mother. We could hear him. We could see him clearly. His mom, we weren't able to see, we weren't able to hear her, so I don't know what her status is, but god willing, she's OK."

He said the child was under his bed frame and mattress.

"He was obviously sleeping," Balboa said. "I can only imagine how many people were in their apartments either sleeping or watching TV or just whatever, just living their lives, unbeknownst to them the building was about to break away.

"If it was not for me and another person being on the backside of the building I have no idea how long it would have been until they found him."

One witness, who was in a neighbouring building, said the collapse felt like a "tornado or earthquake".

The building is located one block away from where Jared Kushner and Ivanka Trump currently live.

Firefighters were also seen using a ladder truck to rescue people who were still in sections of the building that didn't collapse.

One of the residents Barry Cohen, who has lived in the building for three years, said he and his wife prayed as they waited to be rescued.

"It must have been 20 minutes, it felt like a lifetime that we were trapped on the balcony of the building," he told CNN. "When we were waiting to be rescued the building was still shaking. I thought at any minute we could be that same pile of rubble,"

"I thought the whole building was going to collapse so when we were in the cherry picker a feeling of relief came over me," he said.

The building was built in 1981 and had more than 130 units inside.

Officials said they don't know what caused the collapse.