Six more bodies have been found in the shattered ruins of a collapsed Miami-area condominium tower in the past 24 hours, the mayor of Miami-Dade County says.

The latest grim finds bring the confirmed death toll to 18 nearly a week after the building fell.

Nobody has been pulled alive from the mounds of pulverised concrete, splintered lumber and twisted metal since the early hours of the disaster in the oceanfront town of Surfside, adjacent to Miami Beach.

Miami-Dade County Mayor Daniella Levine Cava said 147 people remained missing, feared trapped in the ruins of the Champlain Towers South condo. She said two of the 18 confirmed fatalities were children, aged 10 and four.

“The loss of children is too great to bear,” Levine Cava said on Wednesday.

“Our community, our nation and the world all are mourning with these families who have lost loved ones.”

Officials have said they still harbour hope of finding survivors. Surfside Mayor Charles Burkett said in an interview he had promised families that rescue crews were “not leaving anyone behind”.

Two teams of dogs were helping to scour the pile – one trained to sniff out survivors, the other to detect cadavers.

Investigators have not concluded what caused nearly half of the 40-year-old high-rise to crumple into a heap as residents slept just after midnight last Thursday (local time).

But in 2018, the engineering firm Morabito Consultants prepared a report before a building safety recertification process, finding structural deficiencies in the 12-floor, 136-unit complex that are the focus of inquiries.

As recently as April, the condo association’s president warned residents in a letter that severe concrete damage identified by the engineer around the base of the building had since grown “significantly worse”.

On Wednesday, the relatives of a missing resident, Harold Rosenberg, filed a lawsuit in Florida’s 11th Circuit Court against the Champlain Towers South Condominium Association, Morabito Consultants, and SD Architects PA, a firm the lawsuit says was retained by the association to repair the building.

The lawsuit says the defendants “ignored obvious and shocking warning signs and indications that a catastrophe was imminent” and sought unspecified damages to be paid to estate of Mr Rosenberg, presumed dead, for negligence.

Miami-Dade County State Attorney Katherine Fernandez Rundle said she would convene a special grand jury, apart from any potential criminal investigation, to examine building safety and “what steps we can take to safeguard our residents” from similar disasters in the future.

US President Joe Biden and his wife, Jill Biden, plan to visit the scene in the oceanfront town of Surfside, adjacent to Miami Beach, on Thursday.

-AAP