DALLAS (AP) _ A house fire killed seven members of a family early Thursday, and officials said locked burglar bars kept would-be rescuers out and the victims trapped inside.

One of only three people to survive the fire said he broke out windows with his bare fists after he got out of the house in a futile effort to save his mother, three sisters, two nieces and a nephew.

The flames, which apparently were sparked by an electric heater left on overnight as temperatures dipped into the 30s, spread from a couch through the house about 1:30 a.m., officials said.

''I tried to break out the windows with my hands, but I still couldn't get them open,'' said Leroy Mahomes, 19, who said he awoke to see smoke filling the front room where he slept.

Mahomes, his cut hands bandaged and a thin suit jacket thrown over his shoulders when he returned to the burned-out house after the fire, said his cries failed to awaken most of his family.

The only other survivors were his father, Leroy Sr., who like his son escaped through the only unbarred door, and a niece, 7-month-old Sherveta Mahomes. She was carried out of the home by firefighters and was in fair condition at Methodist Medical Center, according to Battalion Chief Troy England.

The father and son told firefighters that flames near the unbarred door and the bars elsewhere kept them from re-entering the house to help the others.

A neighbor, Melvin Floyd, said he and the two Mahomes men shouted at one of the women to unlock the front door, but she said she couldn't find the keys.

The bodies of Hazel Mahomes and one of her daughters were later found huddled against the locked door, England said. Fire Capt. Levi Whitlow said she had the key to the burglar bars in her hand.

The other five victims, including three children under age 5, were found in the bedroom.

The victims were the Mrs. Mahomes, 48; her daughters, Phyllis Mahomes, 22, Lisa Mahomes, 21, and Charlotte Washington, 23; and their children, Lakiesha Mahomes, 11/2 , Natasha Washington, 2, and Allen Washington Jr., 4.

Ms. Washington was nine months pregnant, a neighbor said.

The fire, declared under control within 90 minutes, gutted the small, wood- frame house. Charred furniture littered the lawn, and several holes peppered the roof.

The Dallas County medical examiner's office listed smoke inhalation as the cause of death for all seven victims. Although the house contained a bracket for a smoke detector, fire investigators said they could not find evidence of a warning device.

The last fatal fire in Dallas in which victims tried to flee but were trapped by burglar bars occurred Nov. 17, said Dallas Fire Department spokesman William Jernigan. The blaze killed an 80-year-old man, his 75-year- old wife and their 49-year-old handicapped daughter.

An ordinance passed in Dallas in 1982 requires all new houses that have burglar bars to be of the type that can be opened from the inside without a key, according to Fire Department spokesman Bill Jernigan.

How the ordinance affects old houses and burglar bars installed before 1982 is unclear and such cases are considered on a case-to-case basis, Jernigan said.