Detectives were last night hunting an arsonist who set a house on fire, killing a young mother, her two daughters, and a teenage baby-sitter.

A top police officer said it was one of the worst incidents he had seen in 31 years service.

Police believe petrol or something similar was poured through the letterbox at the house in Denton Burn, Newcastle upon Tyne.

Ms Lisa Dodgson, 25, and her daughters Amy Louise, two, and Rose Marie Lakey, nine months, were trapped in an upstairs bedroom when the fire started at 5.30am yesterday.

Despite the efforts of firefighters and paramedics, they died at Newcastle's Royal Victoria Infirmary.

Later, babysitter Emma Louise Cater, 16, died at Newcastle General Hospital, having never regained consciousness. The schoolgirl, of Moorvale Lane, Cowgate, had stayed over after babysitting the two children.

Police have launched a murder inquiry and are seeking clues as to why Ms Dodgson was targeted.

Detective Superintendent Derek Storey, leading the inquiry, said: ''This is one of the worst incidents I've dealt with in 31 years with the police.

''This is a tragic incident and it beggars belief that anyone could do this to such young children and their mother. So far we have no motive for such a vicious attack.

''We will be making extensive inquiries into the lifestyle of Lisa Dodgson and we would appeal for any friends, relatives or acquaintances to contact the police.

''We believe the fire was deliberate and an accelerant was used at the front door.''

Fifty Northumbria police officers are working on the case.

Neighbours on the rundown estate were shocked by the tragedy. A young mother said: ''I used to see her all the time with her kids, she just seemed normal. She's lived here about two years.

''It's frightening, I don't know why she was targeted - maybe they will attack anyone.''

Rumours were circulating in the area that Ms Dodgson was 17-weeks pregnant.

Post-mortem examinations are being carried out.