Elderly couple's home gutted by fire

An investigation is underway after a devastating house fire in Sydney's north-west.

Three levels of the Cheltenham house were consumed by the fire that broke out this afternoon.

Neighbours say the blaze started within just seconds, with the front of the house engulfed in flames.

A short time later the roof collapsed under the intense heat.

"I was hanging some washing on the line and I just smelt some smoke. Suddenly there were flames leaping out the windows and glass shattering," neighbour Steve McMahon told 9news.

There were fears for the elderly couple who live at the house and were initially thought to be inside.

But it was later established they were not at the property when the fire started.

Attempts at a search of the house were hampered because the home's internal staircase was destroyed by the fire. There were also fears of asbestos contamination.

The fire was so intense that firefighters were forced to use cherry pickers to extinguish it.

Investigations into the cause of the blaze are continuing.