The work has suspended the search mission, but officials said it should eventually open up new areas for rescue teams to explore.

The decision to demolish the Surfside building came after concerns mounted the damaged structure was at risk of falling, endangering the crews below and preventing them from operating in some areas.

"Our top priority is that the building come down as soon as possible, no matter what time that occurs and as safely as possible," Miami-Dade County Mayor Daniella Levine Cava told a news conference on Sunday.

"Bringing down this building in a controlled manner is critical to expanding the scope of our search-and-rescue effort."

The drilling work was completed on Sunday, with the remaining structure scheduled to come down between 10pm Sunday and 3am Monday local time (12pm-5pm Monday AEST), Miami-Dade Assistant Fire Chief Raide Jadallah told relatives of those missing in the collapse.

Up to 210 rescuers will be poised to restart the search as soon as the site is declared safe after the blast, he said. Rescue officials say that would be 15 minutes to an hour after the detonation.

No one has been rescued alive since the first hours after the June 24 collapse.

The search at the Surfside building has been suspended since Saturday afternoon so workers could begin the drilling work.

So far, rescuers have recovered the remains of 24 people, with 121 still missing. The Miami-Dade Police Department on Saturday night added Graciela Cattarossi, 48, and 81-year-old Gonzalo Torre to the list of those confirmed dead.

Once the structure is demolished, the remnants will be removed immediately with the intent of giving rescuers access for the first time to parts of the garage area that are a focus of interest.

That could give a clearer picture of voids that may exist in the rubble and could possibly hold survivors. Despite the dwindling chance that anyone remains alive in the rubble, officials have pledged to keep looking.

"There's nobody in charge really talking about stopping this rescue effort," Surfside Mayor Charles Burkett told CBS television.

"This rescue effort as far as I'm concerned will go on until everybody is pulled out of that debris."

Officials began considering the demolition at Champlain Towers on Thursday when parts of the remaining building shifted, endangering rescuers and prompting a 15-hour suspension in their work.

Approaching Tropical Storm Elsa added urgency to those plans with forecasts suggesting there could be strong winds in the area by Monday. The latest forecasts have moved the storm westward, mostly sparing South Florida, but National Hurricane Centre meteorologist Robert Molleda said the area could still feel effects.

"We're expecting primarily tropical storm force gusts," Molleda said.

"This July 4 we're reminded that patriotism isn't just about loyalty to country," Levine Cava said as the US marked the Independence Day holiday on Sunday.

"It's about loyalty to one another - to our communities, to those in need whose names or stories we may not know ever, but to whom we are connected by compassion and by resilience."

Australian Associated Press