The city’s deadliest blaze in more than a quarter-century killed at least 12 people — ­including a year-old child — in a Bronx apartment building Thursday night, officials said.

Authorities feared the death toll could rise from the four-alarm fire, which began at 6:51 p.m. and gutted 2363 Prospect Ave. in the Belmont neighborhood near the Bronx Zoo.

“We’re here at the scene of an unspeakable tragedy in the middle of the holiday season, a time when families are ­together,” Mayor Bill de Blasio said during a press briefing at the scene, where bone-chilling temperatures were in the teens.

“Tonight, here in the Bronx, there are families that have been torn apart. This is the worst fire tragedy we have seen in this city in at least a quarter-century.”

The fire stands to be the deadliest New York City blaze since 87 people perished in March 1990 in the Happy Land social-club arson attack, which took place less than a mile away.

About 170 firefighters worked Thursday night to control the blaze in the five-story building just a block from the zoo, authorities said.

“Based on the information now, I’m very sorry to report 12 New Yorkers are dead, including one child as young as 1 year old,” de Blasio said.

“There are four people critically injured who are fighting for their lives. Other serious injuries as well.”

The baby was found in a bathtub — cradled in the arms of her mother, who was desperately trying to protect her child from the flames, law enforcement and FDNY sources said.

Both had perished.

De Blasio warned that more victims could be found as investigators go through the building: “We may lose others as well.”

The mayor praised New York’s Bravest, who got to the scene about three minutes after the first report came in.

“Because of FDNY’s quick response, based on the information we have now . . . at least 12 people were rescued and will survive,” de Blasio said.

Thierme Diallo, who lives on the first floor, ran shoeless out of the burning building into the freezing night in only a bathrobe.

“I was in my bed sleeping . . . and somebody knock on the door shouting, ‘We have fire in the building. Get out! Get out!’ ” said Diallo, a security guard and ­a native of Guinea.

“I don’t know how I get out, no socks, nothing. I left my cellphone there, I took only my wallet. I had to save myself. Then, by the exit, I saw the glass coming down like flames.”

Among the missing is 28-year-old Emmanuel Mensah, who serves in the Army and was home for the holidays, according to his dad, Kwabena Mensah.

“When they rescued [others] . . . they couldn’t find him,” the worried dad said.

Witnesses said they were amazed at how quickly the flames spread.

“It got bad quick,” said a neighborhood resident named Kimberly.

“People were on the fire escapes getting out before [rescuers] showed up, and the firemen got there quick.

“You could see the smoke going through the building,” she said.

Another witness, Xanral Collins, said he feared more children could be among the fatalities.

“A father ran into the building — he couldn’t get in,” Collins ­recalled.

“I saw him screaming, ‘My ­babies are dead! My babies are dead!’ ”

Another witness, Jamal Flicker, said he spotted flames erupting near trash cans.

“It started down where they take the garbage,” he said.

“The smoke was crazy, people screaming, ‘Get out!’ I heard a woman yelling, ‘We’re trapped! Help!’ ”

A second-floor resident named Rafael said he barely escaped.

“The smoke was bad. I had to run for my life out of there,” he said.

FDNY fire marshals didn’t immediately speculate on what could have touched off the inferno.

The blaze was seen shooting up the building’s main stairwell, choking off that one crucial escape path, law enforcement sources told The Post.

Flames accelerated when they reached a natural gas line, sources said.

Investigators with the federal ATF Arson & Explosion unit, the FDNY and NYPD combed through the building late into the night.

“We’re shocked by this loss,” FDNY Commissioner Daniel ­Nigro said. “The fire started on the first floor, quickly spread upstairs into this . . . five-story building with 25 apartments.

“People died on various floors of the apartment. We won’t tell you anything more about it other than they range in ages from 1 to over 50.”

Nigro said the blaze was “without question historic in its magnitude.”

“Our hearts go out to every family that lost a loved one here and everyone that’s fighting for their lives,” the commissioner said.

De Blasio urged New Yorkers to pray for the holiday-week victims: “This evening, hold your families close and keep these families here in the Bronx in your prayers.”

Not including the 9/11 terror attacks, the worst fire in New York City history was the Brooklyn Theater fire of Dec. 5, 1876, which claimed at least 278 lives — and perhaps more than 300.

And the only other city blaze more deadly than the Happy Land arson blaze was the infamous Triangle Shirtwaist Factory fire of March 25, 1911.

Additional reporting by Joe Marino