Massive fire that killed 12 started by child playing with stove, causing 'unprecedented' loss

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

When the family fled the first-floor apartment unit, they left the door open, allowing the flames to spread into the stairwell and shoot up the staircase of the five-story residential building.

"It took the fire so quickly upstairs, people had little time to react," New York City Fire Commissioner Daniel Nigro said at a press conference Friday morning. "Open stairs act as chimneys."

The burned-out halls of the building are seen in video released by New York fire officials on Friday. Soot and ash cover the hallway and stairwell from floor to ceiling.

'This loss is unprecedented'

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

Seven adults and five children died from the fire, including five who weren't able to evacuate the burning building. Another four people who were critically injured in the blaze and hospitalized "are seriously fighting for their lives," the fire commissioner said.

"Last night’s tragic fire, other than 9/11, was the worst loss of life from a fire in our city in almost 28 years," Nigro told reporters. "This loss is unprecedented."

At the moment, he said, there doesn't appear to be anything problematic about the fire safety in the building.

'We may lose others as well'

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 city fire department..

The instability of the charred building prevented firefighters on Thursday night from conducting a full sweep of the building for more fatalities.

"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," New York City Mayor Bill de Blasio said at a press conference Thursday night.

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.

Young children among the dead

Among the children who died from the fire were three young girls ages 1, 2 and 7 as well as a boy whose age has yet to be released, according to the New York City Police Department.

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

The seven adults killed include three women ages 19, 37 and 63.

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.

ABC News' Eva Pilgrim contributed to this report.