Fires burning beneath the rubble of the collapsed condo tower in South Florida are hampering search-and-rescue efforts, Miami-Dade County Mayor Daniella Levine Cava said Saturday morning.

No additional survivors were found overnight in the rubble of the 12-story oceanfront building, which pancaked early Thursday morning, killing at least four and leaving 159 people missing.

Despite the length of time since the collapse, the mayor said the operation is still “very much a search and rescue.”

“We’re facing very incredible difficulties with this fire,” Levine Cava said during a press conference. “It’s extremely difficult to locate the source of the fire.”

She said crews are working under the rubble and using infrared technology to try to find the source of the fires. “Obviously the smoke itself is the biggest barrier right now to proceed in those areas,” she added.

FIre Chief Alan Cominksy said the smoke has been picking up as debris is removed. “We’re dealing with a deep-seated fire, a smoldering fire,” he said. “It’s obviously producing a large amount of smoke.”

That smoke creates concerns for the safety of the rescue crews, who started to need respirators in some parts of the pile. Some ventilation fans are set up, but Cominsky said they are limited because more air can also help the fire expand.