A brief but lethally intense house fire killed seven people - four of them children - and injured two others late Friday night in Southwest Philadelphia.

Six of the dead were found trapped in a basement that had but one exit, City Fire Commissioner Lloyd Ayers said. The seventh, a two-year-old boy, was taken to Childrens Hospital of Philadelphia, where he died.

"We feel real tough about that," Ayers said.

Ayers said the fire did not appear to have been set, but the cause was still under investigation. He said that kerosene heaters were found in several locations in the home and that an absence of smoke detectors appeared to have exacerbated the tragedy.

The blaze was reported at 10:47 p.m. in the 6400 block of Elmwood Avenue. Firefighters were on the scene within three minutes and had the fire under control in 33 minutes, Ayers said, but even that was too late to save six of the victims, who died at the scene. The toddler died just before midnight at CHOP.

Four other persons escaped the fire - two with no injuries, two with burns, Ayers said. Both of the injured were in stable condition at the Hospital of the University of Pennsylvania, he said.

Identities of the victims were not immediately released, but most were believed to have been members of the same family.

The sheer number of casualties prompted Mayor Nutter to visit the scene after midnight, where he viewed the charred, three-story brick structure and mourned the loss of life.

"We are very saddened by this," Nutter said. "Firefighters put forth a tremendous effort, but this fire moved tremendously quickly."

No firefighters were injured. Six residents of an adjacent home were displaced by the fire, Ayers said, and had been taken in by the Red Cross. He said they should be able to return to their home soon.

The fire appeared to have started in the basement, Ayers said, and ate its way up through stairwells and walls to the third story.

"We have not found any smoke alarms at all, which we are very saddened by," he said. "We found kerosene heaters in the basement (and) on the third floor. Outside of that, you see what people have when they are living their lives in a holiday season: pictures, the Holy Bible, clothes stacked...."

One victim was found near the basement exit in the rear of the house, where the fire appeared to have been the most intense. The other five in the basement were not burned, Ayers said, but appeared to have suffered from smoke inhalation while huddling together at the other end of the room.

"It's a horrific sight," he said. Three of the trapped victims were children.

There was no stairway connecting the finished basement to the rest of the house; the only exit was a rear stairway leading from the basement to the outside, he said.

"You need to have a home escape plan, and you can't have a home escape plan from a basement with only one exit. ... We believe there was not proper egress from the basement," the fire commissioner said.

The house was not a rental property.

Ayers said the fire department will supply smoke alarms to any city residents who request them. He urged people to equip every floor of their homes with at least one detector, and to use electric and kerosene space heaters with caution.

"Wintertime," he said, "is a tough time not to be safe."