SURFSIDE, FLORIDA (REUTERS) - Six more bodies have been found in the shattered ruins of a collapsed Miami-area condominium tower, the mayor of Miami-Dade County said on Wednesday (June 30), bringing the confirmed death toll to 18 nearly a week after the building fell.

Nobody has been pulled alive from the mounds of pulverized 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 told a news conference 147 people remained missing and feared trapped in the ruins of the Champlain Towers South condo. She said two of the 18 confirmed fatalities were children, aged 10 and 4.

"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."

He said every day the rubble pile is visibly shrinking, indicating progress. Two teams of dogs are helping to scour the pile: one trained to sniff out survivors, the other trained to detect cadavers.

Investigators have not concluded what caused nearly half of the 40-year-old Champlain Towers South condo to crumple as residents slept in the early hours of last Thursday.

But a 2018 engineer's report on the 12-floor, 136-unit complex, prepared ahead of a building safety recertification process, found structural deficiencies 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 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 Inc; Morabito Consultants Inc; 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 the estate of Rosenberg, presumed dead, 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.

The commander of an Israeli team helping with the rescue efforts said in an interview with CNN that crews were working in the space between collapsed balconies.

Colonel Golan Vach, commander of the Israeli National Rescue Unit, said they had discovered some tunnels large enough for a person, though most of the debris is "very, very tight."

Grand jury inquiry planned

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.

President Joe Biden and his wife, Jill Biden, planned to pay a visit on Thursday to the scene of the tragedy in the oceanfront town of Surfside, adjacent to Miami Beach.

The victims include a father whose wife and children are also missing, a Puerto Rican artist who never let his disability hamper his spirits and a devoted couple of nearly 60 years.

On Wednesday morning, the city of North Bay Village, about 6km south of Surfside, released a statement identifying another victim as Hilda Noriega, the mother of North Bay Village Police Chief Carlos Noriega, calling her the "heart and soul" of his family.

No confirmed signs of life have been detected in the wreckage since last Thursday.

Fire officials have spoken of hearing faint sounds from inside the rubble pile - they acknowledged such noises could come from settling of the ruins - and finding voids deep in the debris large enough to possibly sustain life.

But Miami-Dade fire rescue chief Alan Cominsky said on Tuesday that search personnel faced a daunting task while working in 12-hour shifts in the heat and humidity, hampered by intermittent showers and thunderstorms.

"That building collapsed almost in a footprint of where that building stood - we're talking about 12 stories, with subterranean garages all within that same footprint," Cominsky said.

Debris removed from the rubble pile is being trucked to a collection site to be sifted through by detectives for bits of evidence that will be cataloged and photographed "for future investigative purposes," Miami-Dade Police Director Alfredo Ramirez said.

Hurricane season has arrived in Florida, and the state's emergency management director Kevin Guthrie said at Wednesday's news conference he was making contingency plans should a tropical storm descend on the coast.