Some 159 people remained unaccounted for after the collapse of a residential building near Miami left families clinging to hope as search-and-rescue teams combed through a mountain of debris looking for any signs of life.

The official death toll from Thursday’s disaster stood at four and was certain to rise as rescuers battled smoke, fires and the precarious state of the rubble while working in the south Florida heat.

Although the outlook appeared grim, with one floor of the high-rise stacked on another like pancakes, rescuers continued to search the debris on Friday (local time) in the hopes that pockets had formed, leaving any possible survivors air to breathe.

“We have hope because that’s what our search-and-rescue team tells us, that they have hope,” Miami-Dade County Mayor Daniella Levine Cava told a news conference.

Aided by dogs, cameras and sonar, the teams worked the site on a rotation, with a limited number allowed at any one time to prevent further collapse, Ms Levine Cava said.

Teams from Mexico and Israel arrived to help relieve the locally based crews, many of whom have also travelled to disaster sites around the world.

Atop the pile, some wielded hammers and picks looking for signs of life. Heavy equipment scraped away the top layer.

Below ground, rescuers who entered through the parking garage risked their own lives searching for survivors, occasionally being hit by falling debris, officials said.

Miami-Dade Fire Rescue chief Andy Alvarez recalled that his team once pulled a girl out of earthquake debris in Haiti eight days into the rescue effort.

“You gotta have hope. We’re doing everything we can to bring your family member out alive,” Mr Alvarez told the loved ones of the missing on CNN, pausing as he was overcome with emotion.

The disaster occurred early on Thursday morning as a large section of the 40-year-old high-rise crumbled to the ground.

Security video showed an entire side of the building folding in two sections, one after the other, about 1.30am, throwing up clouds of dust.

The Champlain Towers South condominium is in Surfside, a barrier island town across Biscayne Bay from Miami.

Dozens of people were gathered at a reunification site at the Surfside Community Center – a hectic scene with volunteers running around and people hugging to console one another.

The 12-storey building had more than 130 units, about 80 of which were occupied, officials said.

Rescuers had heard sounds in the rubble overnight, which could have been falling debris or people tapping, Miami-Dade Fire Rescue chief of operations Ray Jadallah said.

US President Joe Biden approved an emergency declaration in the state of Florida and ordered federal assistance to supplement state and local response efforts.

“Our hearts go out to them,” Mr Biden said.

The cause of the collapse remained a mystery.

Satellite data from the 1990s showed the building was sinking about one to three millimetres a year while surrounding buildings remained stable, Florida International University professor Shimon Wdowinski said.

“Either the building is settling into the soil or maybe there is some compromise with the structure, a compromise within the building. We cannot really say,” he said in a telephone interview.

There was insufficient data to show whether the movement continued since then, Mr Wdowinski said.

Officials said the complex, built in 1981, was going through a recertification process requiring repairs.

Surfside Commissioner Charles Kesl told Reuters an email from a building resident in April said the engineering firm Morabito Consultants had been hired to do the building’s recertification. The recertification application, due this year, had not yet been formally submitted, Mr Kesl said.

Kesl said the email, which he declined to share, mentioned the condominium was borrowing about $US15 million ($A20 million) for repairs on balcony railings and repairs that were unclear.

-AAP