New York, New York – Seventeen New York City firefighters were hospitalized – four with serious injuries – battling a fast-moving blaze that tore through a Harlem apartment building under construction Saturday morning, officials said.

The fire erupted on the first floor of the W. 115th Street building, located about a half block from Morningside Park, around 7:30 AM as temperatures dropped to 20 degrees, officials said.

"The fire quickly extended to all floors all the way up to the top floor," FDNY Chief of Fire Operations Thomas Richardson said. "We wound up going eventually to a fourth alarm assignment."

Firefighters smashed windows on all floors in the front and back of the building to ventilate the heat and flames as smoke curled out of the openings.

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Shivering building residents left out in the cold boarded an MTA bus to warm up.

About 200 firefighters and Emergency Medical Service members responded to the blaze. Pockets of fire inside the building were still burning as of 10:30 AM, Richardson said.

"We’re still in the process of checking for further fire spread in the building, but the fire is largely extinguished," Richardson said. "As always the cold weather is a challenge."

Four firefighters suffered serious, but not life-threatening injuries fighting the blaze, officials said. Thirteen other firefighters suffered minor burns and injuries.

All were taken to nearby hospitals for treatment, EMS Deputy Assistant Chief Chris Bilz said.

"In these types of buildings due to the construction fire often does travel in the walls, through the floor and sometimes it will what we call auto-expose with the fire coming out the windows and going into the windows above," Richardson said.

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The building construction may have created openings that helped the fire spread faster through the walls, a source said.

Only a handful of residents were staying in the building because of the construction, the source said.

The cause of the fire was under investigation Saturday.