SURFSIDE, June 30 (Reuters) - Six more bodies have been found in the shattered ruins of a Miami-area condominium tower over the past 24 hours, officials said on Wednesday (Jun 30), bringing the confirmed death toll to 18 nearly a week after the building collapsed.

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, Florida, adjacent to Miami Beach.

Miami-Dade County Mayor Daniella Levine Cava told a news conference that 147 people remained missing and were 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. "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" as teams dig deeper into the rubble.

"We've not gotten to the bottom. We don't know what's down there," he said. "We're not going to guess. We're not going to make a life-or-death decision to arbitrarily stop searching for people who may be alive in that rubble."

Burkett said that every day the mound of wreckage is visibly shrinking, indicating progress.

The search-and-rescue crews consist of about 200 specialists working around the clock in 12-hour shifts, with hundreds more personnel - firefighters, police, engineers, environmental safety experts and others - serving in support roles. Two teams of dogs were helping to scour the wreckage - 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 collapse as residents slept in the early hours of last Thursday.

But in 2018, a report prepared by the engineering firm Morabito Consultants ahead of a building safety recertification process found structural deficiencies in the 12-floor, 136-unit complex that are now 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 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, which 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". It seeks unspecified damages for negligence.

"Given the location of his residence, Harold Rosenberg is likely located at the very bottom of the mountain of rubble that search-and-rescue personnel have only begun to chip away at," the lawsuit said. "Hope is dwindling by the day."

The architect's firm could not be immediately reached for comment.

Brett Marcy, a spokesman for Morabito, said in a statement that the firm's 2018 report "offered detailed findings and recommendations regarding extensive and necessary structural repairs for the condo building".

Both Marcy and Maria Stagliano, a spokesperson for the condo association, said in separate statements that they could not comment on claims made in pending litigation but were working with investigators to understand why the building collapsed.