Firefighters leave after putting out a major house fire on Prospect avenue on December 28, 2017 in the Bronx borough of New York City. Over 170 firefighters respond to the evening fire in which at least 12 persons were killed with others injured. Pic/Amir Levy/Getty Images/AFP

Twelve people were killed, including an infant, and four were critically injured on Thursday in a fire that swept through several floors of an apartment building in the New York City borough of the Bronx, Mayor Bill de Blasio said. The blaze started on the first floor of the brick building and quickly spread upstairs, city Fire Commissioner Daniel Nigro told reporters at a news conference with the mayor. The cause was under investigation. De Blasio said the death toll ranked as "one of the worst losses of life to a fire in many, many years" in the city. Four people were in hospital in critical condition "fighting for their lives," the mayor said.