A 6-year-old boy playing with a lighter started a fire that quickly engulfed an apartment in Northeast Washington yesterday morning, trapping and killing his two younger brothers, authorities said.

The victims, aged 3 and 4, were found in the rear of the burned-out apartment, one on the floor and the other on a bed. Fire officials said the two may have tried to escape out a window but were trapped by heavy metal bars, which also prevented neighbors from entering.

They were the fourth and fifth children to die in four separate fires in the District and Prince George's County in two weeks -- a grim development for fire officials just entering cold-weather months, a time during which house fires are common. The victims ranged in age from 6 months to 4 years.

D.C. Fire Chief Ray Alfred, at an afternoon news conference, again warned parents about the dangers of leaving small children unguarded, even temporarily. Just two weeks ago in the Silver Hill area of Prince George's County, two brothers also playing with a lighter started a fire that killed one of them, a 2-year-old.

Officials said yesterday's fire, reported just after 10 a.m. on the 5800 block of Foote Street NE in the city's Burrville section, was started by Khalig Claggett, 6, who told police he was playing with a lighter behind a sofa in the living room. A woman who was baby-sitting for the children, Loretta Jameson, was in another room watching television and did not notice the fire until it was too late, officials said.

"The deaths of these two children, I think, could have been avoided," Alfred said. "Just being there is not enough. {Parents} are going to have to monitor these children."

The mother, Olena Claggett, 28, left the apartment at 8:30 a.m. to take a high-school equivalency test, and got Jameson, who is about 32, to watch the children, a police source said.

Claggett returned about 12:25 p.m., more than two hours after the fire was put out. Unaware of what had happened, she was prevented from reaching the building by four police officers who quickly surrounded her and placed her in a patrol car.

The apartment, one of nine in the building, was destroyed, and fire officials said the blaze burned for some time before the alarm was sounded. At least a dozen people were in the building, including several other children and at least three elderly women, but all managed to escape uninjured or with only slight burns.

The surviving brother was taken to Children's Hospital, where he was in good condition.

Survivors of the fire described several minutes of terror as the building suddenly filled with thick, black smoke.

One mother threw her 2-year-old son from a third-floor window into the waiting arms of two men, Dennis Jackson and Carter Jacks. Then she jumped. All were uninjured.

Jackson said that Jameson ran out with Khalig in her arms, but did not tell anyone about the two trapped boys, Momar, 4, and Oharon, 3. The building's front door closed behind her, locking shut, making it impossible for Jackson and Jacks to reenter, Jackson said.

Three unidentified men, aware that children were trapped in the bedroom of the first-floor apartment, tried to remove the iron bars with their hands. Herbert Benjamin finally pulled his pickup truck into the yard, tied a rope to the bars and pulled them off.

"One of the guys jumped in, but there was so much smoke and heat, he just jumped right out," Benjamin said.

The first of the three previous fatal fires killed a 1-year-old child, who had been left with his two young siblings in a Northeast apartment Oct. 31. The cause of that fire is under investigation. On Tuesday, while their parents slept nearby, two young children accidentally tipped over a space heater, starting a fire that killed their 6-month-old brother. No school-age children in the District died in fires between October 1989 and September 1990. There are several fire prevention programs aimed at children -- who Chief Alfred said are "fascinated" by fire -- but officials said these must be reinforced by parents.