Photo released by the Miami-Dade Fire Rescue on June 25, 2021 shows a partially collapsed residential building in Miami-Dade County, Florida, the United States.(Photo: Xinhua)

A controlled explosion brought down the unstable remains of the collapsed apartment block in Florida late on Sunday ahead of a threatening , with the confirmed death toll at 24 and 121 people missing.

Video footage showed the rest of the 12-story Champlain Towers South in Surfside, outside Miami, being demolished just after 10:30 pm.

Preparing the site for demolition ahead of the possible arrival of Tropical Storm Elsa early next week had required that the search for victims be halted on Saturday.

Most of the building collapsed in the early hours of June 24, sending up a huge cloud of dust and rattling Americans unprepared for such a deadly urban disaster.

Miami-Dade County Mayor Daniella Levine Cava had announced the schedule at an earlier press conference, saying the operation would use "small, strategically placed explosives."

"The demolition itself is confined to the immediate area around the building," she said.

"However, there is dust and other particles that are an unavoidable byproduct of all types of demolition and, as a precautionary measure, we're urging residents in the immediate vicinity to stay indoors."

With Tropical Storm Elsa rumbling northward through the Caribbean, authorities accelerated the demolition schedule.

County mayor Levine Cava said "bringing the building down in a controlled manner is critical to expanding" the search operation as teams have been unable to delve further due to the risk of further collapses.

The collapse sparked a massive search-and-rescue effort involving engineers and specialists from across the US and as far afield as Mexico and Israel.

Survivors reported being awakened around 1:30 am on  June 24 by what sounded like cracks of thunder that shook their rooms.

"It was like an earthquake," Janette Aguero, who escaped from the tower's 11th floor with her family, told AFP.

Except evacuating dozens of residents, and pulling one teenage boy alive from the rubble, no other survivors have been found, despite deployment of sniffer dogs and cranes for lifting debris.

A 2018 report released by city officials revealed fears of "major structural damage" in the complex, from the concrete slab under the pool deck to columns and beams in the parking garage.

AFP