The cause of the blaze at 2363 Prospect Avenue was not immediately clear, but it began on the first floor and spread rapidly to the third floor, officials said.

“I’m sorry to report 12 New Yorkers are dead, including one child as young as one year old,” Mayor Bill de Blasio told reporters at the scene after the blaze was extinguished by fire fighters.

Four other people were critically injured and fighting for their lives, officials said.

“We may lose others as well,” the mayor said.

The victims range in age from one to more than 50 years old, de Blasio said. A nearby school has been set up as a shelter to house those who were evacuated from the building.

The five-alarm fire broke out in the building in the city’s northernmost borough at around 7:00 pm (0000 GMT Friday) on a bitterly cold night, with temperatures well below freezing.

The New York City Fire Department said on its Twitter account that 15 serious injuries to civilians were reported in the four-alarm blaze and that more than 160 firefighters were on the scene.

City police officer Tiffany Phillips added by telephone that there were an unknown number of fatalities. She said the blaze erupted in a “multiple-dwelling building.”

A photograph posted on Twitter by the fire department showed two fire trucks with two ladders extended to the upper floors of a brick building bathed in flood lights.