Four more bodies were discovered overnight in the rubble of a collapsed apartment building in Florida, authorities said Wednesday, as the search for more than 140 people unaccounted for entered its seventh day.

The official death toll now stands at 16 after most of a building in the Miami-area town of Surfside suddenly pancaked early last Thursday, but hopes are dwindling that the hundreds of rescuers combing the oceanfront site will find anyone alive.

"We've now recovered four additional victims. The number of deceased is at 16. Twelve next-of-kin notifications have been completed, that is four families still waiting to hear," Miami-Dade County Mayor Daniella Levine Cava told a news conference.

She said the personnel in what has become an enormous rescue operation were doing "everything humanly possible, and then some, to get through this tragedy, and we are doing it together."

Colonel Golan Vach, head of an Israeli military unit that specializes in search and rescue operations, told CNN his team had uncovered the bodies as they sifted through the debris, finding what he described as tunnels in the rubble.  

In one case, this space was created between balconies of apartments that pancaked when part of the building collapsed, he said.

"Between them remained a big space of air," Vach said. "We crawled in those tunnels. We called people and unfortunately we didn't find anything."

He said hopes of finding survivors so long after the early morning collapse were "very, very minor. We must be realistic."

No warning 

The cave-in of the 12-story Champlain Towers South building has sparked a search-and-rescue effort involving engineers and specialists from across the United States and as far afield as Mexico and Israel.

US President Joe Biden will on Thursday visit the site with First Lady Jill Biden to ensure state and local officials have everything they need for the rescue effort, and meet with families of the victims.

Residents in the part of Champlain Towers South that remained intact 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. 

But since then, no other survivors have been found, despite huge numbers of rescuers combing the debris with the help of sniffer dogs and cranes.

"There are currently 210 people working on the site. The urban search and rescue team has been augmented by teams from all over the state and all over the world," Levine Cava said on Tuesday.

Experts are looking at possible pre-existing critical flaws in the structure of the apartment tower. 

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

In a letter to residents in April, Jean Wodnicki, the chair of the condo association, described "accelerating" damage to the building since then.

Repairs had been set to begin soon in the 40-year-old building -- but did not come soon enough.