SURFSIDE: President Joe Biden headed to Florida Thursday to meet with families heartbroken by the collapse of a beachfront condo building a week ago, as hopes of finding survivors in the rubble all but vanished.

Biden and First Lady Jill Biden held hands as they left the White House early for the flight to Miami and a trip to nearby Surfside, where the toll in the tragedy now stands at 18 dead, with more than 140 still unaccounted for.

The dead include two children, aged four and 10, Miami-Dade County Mayor Daniella Levine Cava said late Wednesday.

Any loss of life "is a tragedy," she said, "but the loss of our children is too great to bear."

Biden was scheduled to receive a briefing from Levine Cava and other state and local officials, thank first responders and search and rescue team members, and then have a private meeting with families of victims of the catastrophe.

Biden will "offer them comfort as search and rescue efforts continue," the White House said.

Officials said contingency plans were being put in place should a tropical cyclone now swirling in the Atlantic strengthen and strike the southern Florida coast next week, although the National Hurricane Center has said it is "too soon to determine what if any impacts could occur."

Elad Edri, deputy commander of an Israeli search and rescue team, said rescuers had completed a map outlining the bedrooms and other living spaces in the building where residents could have been trapped.

Rescuers made it to an underground parking structure where it had been hoped they might discover people who had been trapped in cars, but found no one, Edri said.

"It's been more than six days from the collapsing," he cautioned, deeming the chances of finding any survivors "low."

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.

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.

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

An October 2018 report released by city officials 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.