Nine people, including eight firefighters, were injured Thursday when a massive fire damaged six buildings in the Bronx, the FDNY said.

The five-alarm blaze broke out at 1136 Olmstead Ave. in Unionport around 3:40 p.m., but quickly spread to neighboring buildings, according to fire officials.

There were “very heavy fire conditions” on two buildings when firefighters arrived about five minutes after the 911 call, FDNY Chief of Department James Leonard said.

“You had tremendous heat upon their arrival,” he said. “They’re going under very intense circumstances to get in there to stop this fire from spreading.”

​One firefighter was rushed to Jacobi Medical Center in serious but stable condition due to the excessive heat and smoke, Leonard said. He has since been upgraded to stable condition, per the FDNY. Seven other firefighters and one civilian suffered nonlife threatening injuries.

Elvin Garcia, 30, who is running for City Council and has an office across the street, said he heard firecrackers before the building went up in flames.

“We heard the pop, pop, pop of firecrackers,” he said. “The house lit up like a piece of paper, that’s how fast it happened. It’s very windy; the fire spread within seconds.”

Guillermo Acevedo, 38, said he also heard an explosion and seconds later the house went up in flames.

“It sounded like a loud firecracker,” he said. “You saw smoke first. It was very quick. Then there was another explosion.”

He said the flames reached 5 to 6 feet above the roof.

“It was kind of crazy, scary,” he said. “This is really bad.”

The fire was finally brought under control around 6:30 p.m., but the cause remains under investigation, the FDNY said.

Dark smoke remained thick in the air Thursday evening as firefighters blasted the burned homes with water.

Two of the buildings will most likely need to be demolished, according to Leonard.

“If you look at...the front of the building, it’s bad. But if you go around the rear of the building, it’s gone,” he said.

Kenaya Arana, 20, was on her way home from work when she realized there was a massive fire on her block.

“I ran. I was scared,” she said.

Arana lives immediately next door to the last house that caught fire, but said her home was spared. The same could not be said for her neighbor’s home.

“There’s nothing but debris, it was just black,” she said of the charred remains of the house. “It’s going to affect a lot of people.”