(CNN) People are feared dead after a possible gas explosion toppled three South Philadelphia row houses and significantly damaged two other buildings on Thursday, officials said.

Firefighters were looking for at least one person in one collapsed building after responding to reports of an explosion and fire at a tenement on South 8th Street about 11:30 a.m. on Thursday, Fire Commissioner Adam Thiel told reporters.

The person was trapped under heavy debris and is believed to have died, he said.

There were unconfirmed reports of possibly one or two other people trapped in the damaged buildings, Thiel said.

"We have to bring in heavy equipment to start de-layering the collapsed building," the commissioner said, describing the scene as still "dangerous and unstable."

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