“Tonight in the Bronx we’ve seen the worst fire tragedy in at least a quarter of a century.
“It is an unspeakable tragedy, and families have been torn apart,” de Blasio said on Twitter.
The fire started on the first floor of a Bronx apartment block and quickly spread upstairs, according to fire officials.
The victims, who died on various floors, ranged in age from one to 50 years old.
At least 12 others were rescued from the building.
Resident Diana Hayes told the New York Times she had escaped with her daughter and pulled a neighbour’s two children from the fire, but that she had to leave other children behind.
“I had one on my front and one on my back. “I couldn’t carry the rest of them,” Reyes told the Times.
The building was built from plaster and brick and was not fireproof, the Times reported.
It was constructed in 1916 and had more than 20 units, according to property records cited by the newspaper.
The 12 confirmed fatalities made the fire the deadliest since an inferno at the Happy Land social club, less than a mile from Thursday’s blaze, which killed 87 people in 1990, the Times said.
Another four people were “fighting for their lives” following the fire, the city’s fire department said on Twitter.
“I want to offer my prayers to all the families who have lost their loved ones this evening or who are struggling.
“I ask all New Yorkers to keep them in their prayers, too,” de Blasio said.
The fire department said earlier it had more 160 firefighters on the scene battling the flames.
De Blasio was briefed on the scene by the Fire Department Commissioner Daniel Nigro.
“This tragedy is, without question, historic in its magnitude.
“Our hearts go out to every family who lost a loved one here and everyone fighting for their lives,” the department wrote on social media, citing Nigro.(dpa/NAN)