A man is in custody and three children are dead after a fatal house fire in Sydney’s west on Sunday morning (AEDT).

Emergency services were called to the blaze on Freeman Street, Lalor Park, about 1am.

Two young boys, aged two and four, were treated at the scene by NSW paramedics.

The children were transported to Westmead Hospital in a critical condition.

Despite the best efforts of medical responders, they died a short time later.

After the fire was extinguished, the dead body of a third child — believed to be a 10-month-old girl, was found in the ashes.

The three children are yet to be formally identified.

A 29-year-old woman was taken to hospital for smoke inhalation.

Officers attached to Blacktown Police Area Command arrested 28-year-old man at the scene.

The man was treated for smoke inhalation and taken to hospital under police guard.

Police have commenced an investigation into the circumstances surrounding the incident.

A report will be prepared for the coroner.