A discarded cigarette caused the fiery car explosion that injured seven firefighters in the Bronx this week, FDNY officials said.

Department Chief John Esposito said the “careless disposal of a lit cigarette” started a trash fire on Fox Street near Westchester Avenue in Longwood around 7 p.m. Wednesday. The blaze quickly spread to a car parked nearby, igniting its gas tank and triggering the explosion, he said.

Of the seven firefighters injured, five were treated for serious burns to their faces and arms and three were admitted to Jacobi Hospital, according to officials. All the firefighters were expected to recover fully, Esposito said.

“If you’re smoking, don’t flick your cigarette butts,” he said in a video message Thursday. “We remind all New Yorkers to properly extinguish cigarettes before throwing them out.”

Firefighters brought the blaze under control by around 8:20 p.m. Wednesday, officials said.

A video posted on the Citizen app appeared to show a ball of flames lighting up the night sky, with smoke billowing as multiple fire engines responded. The FDNY later shared images of the scene, including two charred vehicles as well as burned debris and garbage strewn across the street in front of an apartment building.