The body of a 3-year-old girl who apparently had been dead for some time was found yesterday in a locked Northeast Washington apartment where the girl's mother and sister were lying unconscious, authorities reported.

No explanation for the incident, was immediately available. Dr. James L. Luke, D.C. Medical Examiner said he had no idea of the cause of death of the girl, identified as Asheya Haskel.

The mother, Debra Haskel, 21, and the sister, Fyarie, 4, were being treated last night at D.C. General Hospital and were reported in stable condition.

They were found about 4:30 p.m. yesterday in their second-floor apartment at 226 51st NE by Roy Haskel, Debra Haskel's father. He said that when he obtained a key and opened the locked door to the apartment, he found all four gas burners on the stove alight.

He said the apartment windows were locked and the telephone receiver was dangling from the kitchen table.

Authorities said the dead girl was found on a bed, her mother was lying on the kitchen floor and the older child on a mattress on the hall floor.

After examining the body, Dr. Luke said the child had died "at least a couple of days" ago. He termed it "terribly unlikely" that death was due to natural causes, such as illness, but said he could detect no signs of external injury.

Noting that the stove was on, Luke suggested carbon monoxide poisoning might be the cause of death. He said the gas is generated whenever anything burns, and could have come from a defective stove.

Doctors at D.C. General Hospital said they were checking levels of carbon monoxide in the blood of the mother. But they said that x-ray examination disclosed no evidence that gas inhalation was involved.

A Washington Gas Light Co. spokesman said the apartment and stove were checked after the victims were found. He said no gas leaks were found and the amount of carbon monoxide produced by the stove was "insignificant."

Relatives said they had not been able to reach Debra Haskel since Friday. Her father said he became concerned when she did not pay an expected visit Friday to his house. He said repeated telephone calls to her apartment brought only a busy signal.

Debra Haskel, according to her father, had been living since the end of April in an apartment on 51st Street in a three-story red brick building that is part of a public housing development.

The father said he went to the apartment at 3:30 p.m. yesterday, found it locked and finally returned with a key.

When he entered, he said, he found the contents of his daughter's pocket-book strewn on the floor. Bedroom bureau drawers were open, he said, and clothing had been spilled out.

Homicide detectives went to the scene but said they could make no determination about the death until after an autopsy today.