A massive apartment fire that killed at least 12 people, including a 1-year-old girl found with her mother in a bathtub, was caused by a small child playing with a stove, city officials said.

The blaze started with the child playing with the stove in a first-floor apartment and then raced up the five-story building, causing what de Blasio earlier said was "one of the worst losses of life in many, many years" from a fire New York City.

In addition to the 12 who died, including four children, another four people who were critically injured in the Thursday night blaze were "fighting for their lives," the mayor said.

"This is the worst fire tragedy we have seen in this city in at least a quarter-century," he said. "The search of the building continues so we know that even though it's horrible to report 12 are dead already, we may lose others as well."

The fire broke out in the brick apartment building on Prospect Avenue in the Belmont neighborhood of the Bronx at around 7 p.m. Eastern Time, forcing residents fleeing the flames out into the bitter cold, many without coats.

Firefighters arrived on the scene within minutes, with more than 160 of them braving the frigid temperatures to battle the flames, quelling them completely by about 10 p.m. Eastern Time, according to the New York City Fire Department..

But the instability of the charred building has not allowed firefighters to make a full sweep of the building yet for any more fatalities.

Investigators are working to determine the cause and why the fire moved so quickly. They are looking at the possibility that the blaze burned into a natural gas line in the building, according to city officials.

At least one person on every floor of the building was killed, according to city officials. Five people were pronounced dead on the scene, and seven later died in area hospitals.

"I came out through the window. Yeah, there was smoke everywhere. I couldn't see the door. The door was ... I couldn't see the door. Was covered in smoke already," Matthew Igbinetion, a resident of the building, told WABC.

Among the dead were four children -- three girls ages 1, 2 and 7 and a young boy whose age has yet to be released.

"They were burned, even little kids on the stretchers, burned," a resident of the building told ABC station WABC in New York City.

Four women were killed, including three ages 19, 37 and 63 and one who has yet to be identified. The dead also include four men whose identities are not yet known.

One family is still looking for a missing son, according to WABC.

The fire is the city's deadliest, excluding the Sept. 11 attacks, since the fire at the Happy Land nightclub in the Bronx in March 1990, which killed 87 people.