BOSTON -- Two children from a South Boston family were trapped by smoke and flames and killed in a fast-moving fire early Sunday morning, as the girls' brother screamed at them to jump to the street below.

The fire broke out about 3 a.m. while the family was asleep in their home at 154 W. 6th St. in South Boston, Boston fire officials said. A 14-year-old girl and her 3-year-old sister were on the top floor of their triple-decker.

"I went outside and flames were shooting everywhere," said neighbor Paul Chase.

The girls' mother and brother were able to escape.

Witnesses told NewsCenter 5 that the brother yelled at the girls to jump from the building. "He screamed 'help, help! Throw yourself, throw yourself! She said I can't.'," a neighbor said.

Their bodies were found in a closet, where fire officials think they had tried to seek safety.

"We had two fatalities. They were found on the third floor," said Deputy Chief Robert Calobrisi. Boston police homicide detectives arrived Sunday morning to investigate the cause.

The girls' mother and brother are hospitalized. NewsCenter 5's Shiba Russell reported that the mother's injuries are believed to be more serious.