At least a dozen people including a one-year old baby are dead in what New York City Mayor Bill de Blasio called the worst fire tragedy in the city in a quarter-century.

"In a department that's certainly no stranger to tragedy, we're shocked by this loss," said Fire Commissioner Daniel Nigro.

FDNY

The fire started on the first floor of a five-storey apartment building in the Bronx early on Thursday evening, when most every resident would have been awake.  But it quickly raced up the building to 25 upper units, and within minutes thick black smoke was pouring out the windows.  Around 160 responded to the scene in bitterly cold temperatures, part of a the deep freeze that engulfed much of the US from the Upper Midwest to the Atlantic coast.  Twelve residents were rescued and others fled in their bathrobes and pajamas, but firefighters warn the death toll could go up as they perform a more thorough search of the building.

Investigators said it was "way too early" to determine a cause.  WABC-TV in New York reported that the century-old building is described as "non-fireproof" and is highly combustible.  Buzzfeed news reported that in August,  the New York City Department of Housing Preservation and Development had sent a notice of violation to the building's owner about defective smoke and carbon monoxide detectors in a first-floor apartment.

"In the middle of the holiday season, a time when families are together, tonight here in the Bronx there are families that have been torn apart," said Mayor de Blasio.  "This is the worst fire tragedy we have seen in this city in at least a quarter century."

"Based on the information we have now, this will rank as one of the worst losses of life to a fire in many, many years," he added.