A controlled explosion brought down the unstable remains of the collapsed apartment block in Florida late Sunday ahead of a threatening tropical storm, 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 local time (0230 GMT).

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.

Authorities had said the unstable remaining structure posed a threat to search and rescue teams still at the scene, though hopes of finding anyone alive have diminished.

The video footage of the demolition showed smoke, dust and other debris blowing away from the site after the building came down following two loud booms.

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."

Storm threat

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.

President Joe Biden visited the scene last week and met grieving relatives and rescue workers, hailing their resilience and strength.

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

Survivors reported being awakened around 1:30 am (0530 GMT) Thursday 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.

Rescuers who arrived in the moments after the tower came down helped evacuate dozens of residents, and pulled one teenage boy alive from the rubble. No other survivors have been found, despite deployment of sniffer dogs and cranes for lifting debris.

Experts are looking at possible pre-existing critical flaws in the building's structure. 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.

In a letter to residents in April, Jean Wodnicki, chair of the condo association, described "accelerating" damage to the 40-year-old building since then, and repairs had been set to begin soon. Several cases of Covid-19 had been found among searchers. One team was pulled out and medical protocols put in place to isolate anyone affected.