Neighbor Sontaya Perry, 22, said she could hear the children inside the house screaming as the fire raged from the windows before dawn. She said she tried to get in, but the wooden steps leading to the door were in flames.
"They were screaming, and five minutes later they stopped screaming," Perry said.
On the street outside, two boys were begging for someone to help for their brothers and sisters, she said.
The victims, ages 2 to 7, were all found on the second floor, authorities said. One was dead on arrival at West Penn Hospital and two others died in the emergency room, said Stephanie Waite, a hospital spokeswoman. The Allegheny County Medical Examiner's office said it had the bodies of a boy and a girl.
Family members identified some of the victims as Desekiah (age 2), Cedonia (age 3) and Daekia Holyfield (age 6), reports CBS station KDKA-TV. Two other children who were killed, Azquel (age 4) and Andre Rankin (age 6), have been described as friends of the family.
Two eight year-olds, Javon Irwin and Huedan Chmablis, managed to escape the fire, KDKA-TV reports.
Two other children were able to escape without injury.
The victims' grandfather, Adrian Artemus, spoke to KDKA's Bob Allen.
"We're just taking it as it comes," said Artemus. "I just recently lost my dad and my uncle. I'm a wreck at this point and I'm just trying to hold on for my daughter."
Police said a teenage baby sitter was supposed to be watching the children, from two different families, while their parents were out for the night. A police statement said three of the five children who died lived in the house and the two others lived elsewhere, but it did not clarify their relationships.
The baby sitter had not been located as of late morning and police only knew her nickname, which they didn't disclose, Police Chief Nate Harper said.
"We have a lot of people out there working hard. ... Hopefully we'll have answers in a short period of time," he said.
Tuesday morning, investigators were inside the gutted and blackened house skeleton of the home, examining what remained and trying to determine the cause of the fire, which officials believe started on the home's second floor.
Officials said flames had been shooting from all three floors of the building in the city's East Liberty section when firefighters arrived around 1:20 a.m. The blaze also damaged a vacant building next door.