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BELMONT, the Bronx — New York City’s deadliest fire in a quarter century was started by a small child playing with a stove, Fire Commissioner Daniel Nigro said Friday.

Twelve people, including five children, were killed and four others critically injured when a fire ripped through a five-story, walk-up apartment building at 2363 Prospect Ave. in Belmont Thursday night, Nigro said.

A 3-year-old boy with a history of playing with stove burners started the fire in a first-floor unit, according to Nigro.

The child’s mother heard him screaming, and was able to rush him and a 2-year-old out of the unit, but she left the door open, the fire commissioner said.

“This fire quickly spread up the stair,” Nigro said. “The stairway acted like a chimney. It took the fire so quickly up stairs, people had very little time to react. They couldn’t get back down the stairs. Those that tried, a few of them, perished.”

The lessons learned from the deadly incident are that children cannot be left unattended, and when fleeing a fire, people should always close doors behind them, Nigro said.

Fire crews arrived at the building a little more than three minutes after the blaze started, but it was too late.

Five people were later found dead inside, Nigro said.

Four people from a single family are among the dead, family members said.

A 1-year-old girl and 63-year-old woman who are related also died, police sources said.

The fire has been called the worst fire tragedy the “city has seen in at least a quarter century,” by Mayor Bill de Blasio.

Four people in critical condition continue to fight for their lives Friday, and 10 other civilians also suffered injuries.

About six firefighters and one EMT suffered minor injuries, officials said.

Because of the number of lives lost, Nigro called the fire, “historic in its magnitude.”

The investigation into the fire, including whether or not the building had working smoke detectors, continues.

NYPD tweeted out the number to call for anyone looking to contact a loved one who may have been affected by the blaze: “If you’re unable to reach someone who may have been impacted by the five-alarm fire at East 185th Street and Prospect Avenue in the Bronx, call 311 or 212-639-9675 from outside NYC.”

Correction: A high-ranking fire official initially told PIX11 the fire was suspicious. Officials later said it was caused by a child playing with a stove. The story has been updated with the correct information. In addition, the story initially misreported information about building violations.