Updated

At least 12 people are dead – including eight children – after a fire erupted in a three-story Philadelphia duplex early Wednesday, officials say.

The blaze happened at a property owned by the Philadelphia Housing Authority and the four smoke detectors inside the building were not functioning, according to Philadelphia Deputy Fire Commissioner Craig Murphy.

"This is without a doubt one of the most tragic days in our city’s history," Mayor Jim Kenney said. "The loss of so many people in such a tragic way. Please keep all these folks and especially these children in your prayers. Losing so many kids is just devastating."

Philadelphia firefighters respond to the scene Wednesday morning.

Philadelphia firefighters respond to the scene Wednesday morning. (FOX 29 )

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Murphy said upon arrival around 6:40 a.m., firefighters found heavy flames coming out of the second floor at the front of the building.

He said eight people – the number who live in the building's first-floor unit – managed to self-evacuate, while a combined 18 people occupied the property's other dwelling upstairs.

"That is a tremendous amount of people to be living in a duplex," Murphy said.

But Kenney noted that "you don't know the circumstances of each and every family and maybe there were relatives or people that needed to be sheltered. 

"Obviously the tragedy happened, and we all mourn for it, but we can’t make judgment on the number of people in the house because sometimes people need to be indoors," he told reporters gathered near the scene of the blaze.

Firefighters work at the scene of a deadly rowhouse fire Wednesday in the Fairmount neighborhood of Philadelphia. 

Firefighters work at the scene of a deadly rowhouse fire Wednesday in the Fairmount neighborhood of Philadelphia.  (AP)

Officials said 13 people died in the fire on Wednesday morning, but reduced that number to 12 hours later. 

In a tweet, the Philadelphia Fire Department said it "took 50 minutes to place the fire under control." Murphy added that firefighters encountered heavy flames in a kitchen area on the second floor of the building.

Fire crews arrive at the scene of a fatal fire Wednesday morning in Philadelphia.

Fire crews arrive at the scene of a fatal fire Wednesday morning in Philadelphia. (WPVI)

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"I’ve been around for 35 years now and this is probably one of the worst fires I’ve ever been to," he said. "We are working with the fire marshal’s office, we plan on and intend on getting a cause for this fire. We plan on making sure that this tremendous loss of life did not happen in vain." 

A neighbor who lives near the scene of the fire described it to CBS Philly as being like "your worst nightmare."