Four people were injured and more than 100 firefighters responded after an explosion on the top floor at a high-rise condo building in Sunny Isles Beach, Florida, Friday.

The incident happened just before noon at the Chateau Beach Residences at 17475 Collins Avenue.

Multiple fire rescue units responded and several people were seen being treated for injuries outside the building.

One victim was airlifted to a nearby hospital from the scene by Miami-Dade Fire Rescue. Another three people were injured, and two firefighters suffered minor injuries.

Two other people were trapped in an elevator but weren't injured, fire rescue officials said.

"There was no life-threatening injury, we had one victim that was a burn victim and the rest had injuries that were not life-threatening," Sunny Isles Beach Mayor Bud Scholl said.

Footage showed parts of the top of the building badly damaged and debris scattered across the roof and in a rooftop swimming pool.

Officials with the City of Sunny Isles Beach told NBC 6 there was a natural gas explosion in a boiler room which sent glass and debris out of the building. The debris fell on patrons at a hotel next door as well as beachgoers.

The building was under construction at the time and the only people inside were about 20 workers, officials said.

One of the workers said he heard two explosions.

"As we were getting out material, we heard the first explosion, then when I got in front of the elevator that's when the second explosion hit and the elevator dropped," Cleo Fleming said. "What was I thinking? I'm gonna die, anything like that, but it didn't happen."

Witnesses said they were stunned by what they saw.

"All of a sudden I heard this big explosion and you see a piece of concrete just come yanking down from the air," Aldo Mottolese said.

"Basically the wall cracked and you heard the big boom and everything just fell down," Leidy Garafalo said.

Collins Avenue was closed between 172nd Street and 178th Street as a result of the incident. Motorists were advised to avoid the area.

Fire rescue officials said the incident remains under investigation.

Check back with NBC 6 for updates.