Five people were killed and 23 others injured Thursday in the collapse of a six-story building under construction in Cartagena, Colombia, the city's mayor said.

"As of now: 23 injured, two rescued unhurt and five mortal victims. Rescue operation continues without pause," Manuel Vicente Duque wrote on Twitter.

The disaster took place around 11:30 am at a building site in the Caribbean port city's Blas de Lezo neighborhood, where as many as 30 workers may have been trapped under the structure.

Noting that the collapse took place near lunch time, the mayor told EFE earlier that officials were holding out hope that some of the workers were away from the site when it happened.

One of the workers, Alberto Sorte Velasquez, said that he and his brother were working on the second floor at the time of the accident.

"Everything happened suddenly. We got down immediately," he recounted. "We managed to reach the light by pushing rubble out of the way."

Sorte said that from what he could see, only seven workers were able to get out.

Area residents had begun pulling workers from the rubble when first responders arrived at the scene.

"We've activated all the rescue agencies, who are already working in the zone," Duque said.

The Colombian Red Cross also dispatched 27 volunteers and two vehicles to the site.

Cartagena's police chief said that the accident appeared to be the result of "poor engineering calculations."

"There is a fracturing of the cement that made the structure fall down," Gen. Luis Poveda said.