A 12-story oceanfront apartment block in Florida partially collapsed early Thursday, killing at least one person and leaving dozens more unaccounted for as teams of rescuers combed the rubble for possible victims.

Video footage posted online showed a large portion of the building in the town of Surfside - just north of Miami Beach - reduced to rubble, with the apartments' interiors exposed.

It was unclear what caused the building to collapse, nor how many people were inside at the time, since it was occupied by a mix of full-time and seasonal residents and renters according to Miami-Dade County Commissioner Sally Heyman.

"It's hard to get a count on it," Heyman told CNN. "It is 51 people who were supposedly, supposedly, residing there at the time (and) have not either called out or had people call in to reach them."

"But you don't know between vacations or anything else. So we're still waiting. And unfortunately, the hope is still there, but it's waning," she told CNN.

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 1.30am, evacuating 35 people from the building. 

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.

She said the search phase of operations had now concluded and that the focus was now on the recovery of possible victims amid the rubble, in a massive operation assisted by drones and dogs and involving both 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," mayor Burkett he said on NBC's Today show.

Surfside's town manager Andrew Hyatt told the news conference that search operations could last a week.

At least four Argentinians - three adults and a six-year-old girl - were among those unaccounted for, according to the country's foreign ministry.

'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.

Miami resident Nicolas Fernandez, 29, said he has yet to hear from friends who were staying overnight in a unit that his family owns in the building. 

"The one day that they decided to stay there overnight is the one day that this happened," he told AFP.

Fernandez 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. 

Miami-Dade mayor Daniella Levine Cava tweeted out a message of support, writing: "My prayers are with all those impacted by this horrific tragedy and with their families." 

"We are so grateful to @MiamiDadeFire, @MiamiBeachFire and all the first responders on the scene – may they stay safe while working to save lives."