A charity last night urged the Government to tighten up its advice to parents about leaving their children with young babysitters following an horrific house fire in the North-East.

The appeal, from the National Society for the Prevention of Cruelty to Children (NSPCC), came after the blaze claimed the lives of a sister and brother.

Investigations will continue today into the cause of the mystery blaze that killed Caitlin Scully, three, and 23-month-old Joseph Sands.

They were trapped in a bedroom in the family's mid-terraced council house in Middlesbrough, in the early hours of yesterday.

Two 13-year-old babysitters, who were downstairs, escaped from the fire in Charlbury Road, Pallister Park. The girls, who are neighbours' children, were shocked but otherwise unhurt.

Firefighters and paramedics fought in vain to revive the two infants.

Neighbour Simon Mallory, 24, who tried to save the children but was beaten back by the flames, said the memory would haunt him for ever.

"I will never forget hearing the children screaming and crying," he said.

"I could only get to the top of the stairs and then I was forced back. There was nothing I could do and I couldn't get any further."

The dead children's mother, 22-year-old Marie Scully, who is four months pregnant with her third child, was at work as a door supervisor at a Middlesbrough pub when the fire broke out.

Their father, Robert Sands, also 22, was in Derby where he is working as a labourer.

Ms Scully said: "I was taken to the police station after arriving home to be told both children had passed away.

"Then I was taken to the hospital and I had a chance to see the children a few times before Rob arrived. They died because of all the smoke."

She added: "We were told Caitlin had been covering Joseph up with a blanket, and when the fire brigade went in they thought there was only one child. The neighbours told them there was a second child in the house.

"Joseph died after Caitlin. Firemen managed to get Joseph's heart going again, but he didn't really stand a chance. It's so horrible and I just don't know what to do."

Home Office forensic scientists assisted Cleveland Police and Cleveland Fire Brigade investigate the cause of the fire, which last night remained a mystery, although arson was ruled out.

Divisional Fire Officer Alan Gill said the fire was contained in the rear bedroom where the children were both found. He added: "The crews did all they could to save the children."

Smoke detectors at the house have been taken away for examination.

The NSPCC called for clear guidelines on how young a child should be to be left to babysit.

A spokeswoman for the charity said that at present the law does not state an age when young people can babysit, although no one under the age of 16 can be charged with neglect or ill treatment of a child left in their care.

Parents, therefore, remain responsible and can be charged themselves should any harm come to their child.

For this reason, the NSPCC does not recommend asking anyone under 16 to look after a child.

The spokeswoman said: "We want the Government to give clear guidelines because parents are really confused and we know that.

"And our experience also shows parents sometimes don't know just how vulnerable those children are."

Last night, a spokesman for the Department of Health, which has responsibility for such issues, was unable to comment on the guidance available.

But he added it would look at any proposals made by the NSPCC.