The discovery of four more victims in the rubble of a collapsed apartment building in Florida has raised the death toll to 36, a fire official said, as a ramped-up search effort faces new threats from severe weather.

Miami-Dade Assistant Fire Chief Raide Jadallah announced the new death count to family members during a closed-door morning briefing on Tuesday, according to video posted on social media. He said rescuers have also been locating more human remains.

Mr Jadallah said there was a two-hour delay early on Tuesday as a result of lightning. He added that workers have removed 124 tons of debris from the pile.

At the site of the collapsed building on Tuesday morning, power saws and diggers could be heard as workers in yellow helmets and blue jumpsuits sifted through the rubble for a 13th day.

Stiff winds of 20mph with stronger gusts blew through the area as grey clouds from Tropical Storm Elsa’s outer bands swirled above.

Bands of rain are expected in Surfside as Elsa strengthens with potential to become a hurricane again before making landfall somewhere between Tampa Bay and Florida’s Big Bend and crossing northern Florida.

The search crews can work through rain, but lightning from unrelated thunderstorms have forced them to pause at times, and a garage area in the rubble has filled with water, officials said.

Up to 113 people remain unaccounted for, though only 70 are confirmed to have been inside the Champlain Towers South building in Surfside when it collapsed in the early morning hours of June 24, Miami-Dade County mayor Daniella Levine Cava said.

“Active search and rescue continued throughout the night, and these teams continue through extremely adverse and challenging conditions,” she told reporters. “Through the rain and through the wind they have continued searching.”

Crews got a big boost when the unstable remaining portion of Champlain Towers South came down on Sunday.

The demolition — prompted by fears that the structure could fall — allowed rescuers into previously inaccessible places, including bedrooms where people were believed to be sleeping at the time of the disaster.

“The site is busier and more active now than I’ve seen it since we began, now that the damaged building is down,” Surfside mayor Charles Burkett said, adding that heavy equipment was now able to move freely around the site.

“We’re actively searching as aggressively as we can,” Miami-Dade Fire Chief Alan Cominsky said, but added that crews have detected no new signs of survivors.

“We are not seeing anything positive,” he said.

No one has been rescued alive since the first hours after the collapse, but rescuers are still holding out hope of reuniting loved ones.

“We continue to remain focused on our primary mission, and that is to leave no stone unturned and to find as many people as we can and to help bring either some answers to family and loved ones or to bring some closure to them,” City of Miami Fire Rescue Captain Ignatius Carroll said.