NEW YORK >> A preschooler toying with the burners on his mother’s stove accidentally lit New York City’s deadliest fire in decades, turning an apartment building into an inferno that killed a dozen people as smoke and flames swept up the stairwell in minutes and blocked the main route to safety, the fire commissioner said today.

The 3 ½ year-old-boy, his mother and another child were able to flee their first-floor apartment. But they left the door open behind them, and it acted like a chimney that carried Thursday’s fire out of the apartment and through the five-story building, Fire Commissioner Daniel Nigro said. At least 20 of their neighbors scrambled out via fire escapes on a bitterly cold night, but others could not.

“People had very little time to react,” Nigro said. Although firefighters arrived in just over three minutes, “bravely entered the building and did everything they could — we did save a number of residents — this loss is unprecedented.”

Twelve people died, including girls ages 1, 2 and 7 and a boy whose age was not given, officials said. Four other people were fighting for their lives.

Fernando Batiz said his sister, Maria Batiz, 56, and her 8-month-old granddaughter, were among the dead, though the baby’s mother survived.

“They couldn’t escape … The smoke, I guess, overcame her — everything happened so quick,” said Batiz, 54. He said his sister, a home care attendant, was a selfless person who had helped him when he was homeless.

“I don’t know what to think. I’m still in shock,” her shaken brother said Friday.

Excluding 9/11, it was the deadliest blaze in the city since 87 people were killed at a social club fire in the same Bronx neighborhood in 1990. A fire in a home in another part of the Bronx killed 10 people, including nine children, in 2007.

Thursday’s fire broke out just before 7 p.m. in a century-old building near the Bronx Zoo. Its roughly 20 apartments are home to people from countries ranging from the U.S. to the Dominican Republic to Guinea.

About 170 firefighters worked in 15-degree weather to rescue dozens of people.

Residents described opening their front doors to see smoke too thick to walk through and clambering down icy fire escapes with children in hand. Some escaped barefoot or in their nightclothes.

Huddled in a deli on the block with her family, Crisbel Martinez, 10, cried Friday as she recalled her escape from her fifth-floor apartment with her three older brothers.

One brother’s girlfriend was coming into the building when she saw smoke, called him and called police. With their mother at work, the siblings checked and saw the smoke.

“Then we got changed and went through the fire escape,” Crisbel said. She had spent the night at an aunt’s house.

Twum Bredu still didn’t know Friday afternoon what had become of his brother, Emmanuel Mensah, 28. He was staying with a family that had escaped the fire safely, but no one could find Mensah, despite checking four hospitals. Still, his family kept looking, and hoping for word of him.

“That’s my prayer,” said Bredu, 61.

Four families sought emergency housing Thursday night from the Red Cross, but the organization expected to get more requests in the coming days, spokesman Michael de Vulpillieres said.

Catastrophic fires at the turn of the 20th century ushered in an era of tougher enforcement of fire codes. But the building was not new enough that it was required to have modern-day fireproofing, like sprinkler systems and interior steel construction.

Building owner Annal Management Co. declined to comment Friday.

The boy who accidentally started the fire had played with stove burners before, Nigro said.

He noted that it’s not rare for children to start fires. The Fire Department gets 75 or more referrals a year to a program that aims to educate children fascinated with fire about its dangers.