A catastrophic fire tore through a row house in Philadelphia’s Fairmount neighbourhood before sunrise on Wednesday.

The fire, which had killed 12 people, including eight children, in one of the nation’s deadliest residential fires in decades.

The victims were yet to be identified by the authorities, but relatives and friends said they included two mothers and their children.

Some attended a nearby elementary school and one possibly as young as two.

Together, they occupied the top unit of a three-story brick home where at least four smoke detectors weren’t working.

Investigators could not say what sparked the 6:30 a.m. blaze or why it became deadly so swiftly.

Federal agents were expected to join Philadelphia fire marshals in what was likely to be a complex and lengthy probe.

The loss gripped the city and rippled across the country, drawing condolences from Governor Tom Wolf to first lady Jill Biden.

In Fairmount, a stately neighbourhood just northwest of Centre City, residents who were shaken awake by the sound of sirens and a woman screaming soon embraced on the sidewalk as first responders choked back tears.

Philadelphia Police Cpl. Jasmine Reilly said an initial 911 call was answered just after 6:30 a.m.

Fire crews arrived on the scene within five minutes. They found flames shooting out the second-floor windows, and the heavy fire had already run up an open stairwell into the third, said Craig Murphy, first deputy commissioner of the Philadelphia Fire Department.

(dpa/NAN)