High winds and the collapse of the building's rubber roof complicated efforts to rescue residents and control the blaze, said Bethlehem fire Lt. Joe Chernaskey.
"The intensity of the fire ... was too much for them to advance too far," Chernaskey said of the firefighters.
The 4:30 a.m. fire destroyed all three homes in the building but appeared concentrated in the middle residence. Each home was occupied, but officials did not immediately know how many people were inside or how many escaped.
A woman and five children lived in the middle, red brick row house, neighbors said.
Hector Lugo, 27, who lives three doors away, was awakened by the sound of the woman screaming "Help! Help! My kids are inside."
He saw a teenage boy on fire jump from a second-floor window and watched as firefighters doused the flames from him, he said.
When firefighters opened the front door, flames exploded outward, forcing people to run for cover, Lugo said.
The resident who jumped was hospitalized for burns, while three firefighters and a paramedic were treated for minor injuries, authorities said.
The victims' names were not released. Northampton County Coroner Zachary Lysek did not immediately return a call seeking comment.
The homes, in a working-class neighborhood in South Bethlehem, are within sight of the former Bethlehem Steel blast furnaces, where an $800 million casino complex is being built.
Rafael Medina, 64, who lives next-door to the charred building, said the woman knocked on his door screaming for help. He tried to get inside, but the heavy smoke blocked his way.
Medina frequently saw children from the family outside playing.
"They're nice kids," he said.
Myriam Rivera, who baby-sat two of the young children in the house, said one of the girls was turning 8 years old on Monday.
Rivera said she visited the mother at Lehigh Valley Hospital, where she said the teenage boy who jumped from a second-story window was being treated for burns.
Another neighbor, Aracelis Rodriguez, said it was tough explaining the fire to her 8-year-old daughter, who played with the children in the burned house. "She started crying and asked why it happened," Rodriguez said.
Action News web producer T.J. Alexander contributed to this report.
