At least 13 people are dead, including seven children, after a large house caught on fire in Philadelphia's Fairmount neighbourhood on Wednesday morning.
One child and one adult were taken to local hospitals, fire authorities said at a news conference. Eight people managed to escape the building.
Philadelphia Mayor Jim Kennedy told reporters to "keep those babies in your prayers", but did not provide their ages.
Deputy Fire Commissioner Craig Murphy added: "I've been around for 35 years now and this is probably one of the worst fires I've ever been to.
"I don't have the words for how we're feeling right now as a community and as a department."
The blaze occurred in a building owned by the Philadelphia Housing Authority, which had converted it into two apartments.
Twenty-six people lived in the terrace house, with 18 people living in the second unit alone.
"That is a tremendous amount of people to be living in a duplex," Mr Murphy said.
Authorities said none of the four smoke detectors in the building were functional.
Firefighters arrived at the three-story house at 6.40am local time and reported heavy fire coming from the second floor. It took 50 minutes to extinguish the blaze.
The origin and cause of the fire are currently under investigation.
