UPDATED 4.54pm: Neighbours came to the rescue in a house fire in which a young boy died in Hamilton this morning.

The fire was well advanced when fire fighters arrived at Karen Crescent in Dinsdale shortly before 11am.

The three-year-old died at the scene, and three others have been taken to hospital, including his mother who received CPR and is in a critical condition.

Waikato Fire Area Commander Roy Breeze believes a neighbour broke a bedroom window to rescue a teenager and another youngster, before handing them to two or three helpers outside.

"We believe a neighbour actually broke a beacon window and climbed in and rescued a teenager and a baby and handed them out the window to two or three helpers that came to his assistance."

Breeze says the fire was already well advanced when fire services arrived.

"The end room that looked like the main bedroom was well involved to the stage where there was holes in the floor, so it was totally involved, and the fire had spread throughout the house."

Mr Breeze says the cause of the fire is yet to be confirmed.

Three other people have been taken to Waikato Hospital.

A 35-year-old woman is in a critical condition while a four-year-old and an 18-year-old are being treated for smoke inhalataion.

A fire safety investigator is at the scene this afternoon.

The house has been extensively damaged.

The fire was well advanced by the time the fire service arrived.