At least 13 people, including seven children, were killed in one of the worst fires seen in Philadelphia in years after smoke detectors failed to go off in a public-housing apartment building, the city's Fire Department said.
Eight people managed to escape the building through one of the two exits.
The blaze, which occurred in the popular museum district of Fairmount, was one of the deadliest in recent memory in America's sixth-most populous metropolis.
A child and an adult were taken by paramedics to nearby hospitals. The smoke detectors in the building failed to activate, fire officials said.
"This is without a doubt one of the most tragic days in our city's history, the loss of so many people in such a tragic way," Mayor Jim Kenney told reporters.
Philadelphia Fire Department deputy commissioner Craig Murphy said the fire was the worst he had seen in 35 years on the job.
He told reporters that the toll was "dynamic because there is still an ongoing recovery effort inside," adding that two additional people were rushed to hospital.
"We're in the process of investigating this to the highest level that we can. We're incorporating all of our resources."
Mr Murphy said that there were four smoke detectors in the building but none of them had been operating. The building is owned by Philadelphia's public housing authority.
Officers found "heavy fire" coming from the second floor of the three-storey terraced house at the 800 block of North 23rd St when they arrived at 6:40am, the fire department said
"It took 50 minutes to place the fire under control," it said in a tweet.
About 26 people had been living in the building, eight on the first floor and 18 across the second and third floors, according to Mr Murphy.
It wasn't clear how many people were in the building during the fire.
"Obviously the tragedy happened and we all mourn for it. But we can't make judgment on the number of people in the house," added Mayor Kenney.
The Philadelphia Inquirer quoted police as saying that the three-storey home had been converted into two apartments.
Neighbours reached by local media said they had been shaken by the fire.
Bill Richards, who lives nearby, told the Inquirer that shortly before 7am he had heard a woman scream "Oh my god, oh my god."
"It's totally devastating and upsetting," he said.
